I want to create some universe/backstory that makes an existence of a feudal form of gevernment for a space empire reasonable. Under what conditions it would be the best/acceptable model of government? Or what causes the government type to not change to another (more efficient) form? Why feudal and not republic/democracy/absolute monarchy/dictatorship/tyrany?

I would prefer to avoid space navigation reasons (like the empire is very big and it takes thousand years to reach the far end).

The structure of the space empire should be, more or less, like this (if possible):

- emperor (probably also a head of a noble house, so he is not just one person but has a whole family and core followers behind)

- 5-6 major noble houses (and possibly some smaller ones; but still they are not single nobles fighting each other, they act (usually) as a united noble house, at least that how it appears on the outside)

- some powerful guilds/organizations (probably interested in commerce or other goals that do not include usurping the imperial throne)

- common population

Relatively slow and inexpensive space flight allows colonization of planets and star systems but requires a large degree of autonomy (nobody can come to help), ideally suitable for making every planet a fiefdom.
For example, nuclear-powered drives pushing large ships close to lightspeed would allow regularly scheduled caravans between, say, Earth and Alpha Centauri.

Apart from the planetary rulers, equally (or more) important lords could command fleets and space stations in deep space, while the most important nobles could rule one or more star systems (obviously losing sleep on what's the best location for their own space navy).

Fast communication would allow large organizations to function and keep the empire together.

I don't know why you are averse to "space navigation" excuses, but you don't need a huge empire with flight times of thousands of years; putting help even a few weeks away is enough for most military engagements, catastrophes, etc.

You could also switch an empire that matured with slow travel to FTL propulsion, with a likely outcome of global war between planets and/or noble houses and/or guilds (when there is a way, there is a will).

Requirement for a large degree of autonomy alone is not enough to justify feudalism. Federation of losely aligned planets can do equally as well (and has an upper hand in terms of commerce/economy/research).

I think we should follow the direction of:

- what is the advantage of feudalism over other systems?

- what caused the occurence in medieval feudalism in Earth's history (note that before there was Democracy(Greece) and Republic(Rome) then it switched to feudalism (medieval europe) and then is switched to democracy/republic again (modern times))? It was not a linear evolution of the government types (more like a jojo :)), it seems something strange/unusual caused it. Maybe the scientific/whatever regress after the fall of the Roman Empire?

Colonizing planets (i.e. building ships and recruiting people) is likely to be a job for billionaires or megacorporations or criminal syndicates, which are then likely to "own" their colonies and embrace feudalism both downwards (supply chains of smaller corporations, criminal gang hierarchies, delegation to ambitious managers and friends, etc.) and upwards (diffidence towards other colonies, respect for stronger parties, maximum independence from the central government, etc.), as a white collar counterpart of the medieval tribal warlords who built feudal nations in real history.

An empire which is unable to impose its authority is likely to institutionalize informal feudal trends, leading to much more feudalism at smaller scales (e.g. a city) where democratic governments would work well enough.

A different good reason for feudalism is that the only things space colonies have in common are likely to be alliances against outside enemies, free trade agreements and other treaties; if there is a war against enemies of the Empire, contributing armed forces to a common cause a large part of economy and daily life.
In such a situation, feudalism offers the fractal organization structure that allows the best compromises between defending one's home and contributing to longer term war campaigns at multiple scales; joining a fleet of peers and submitting to a commander is acceptable, but nobody wants to be the planet that a distant central Fleet Command abandons "strategically" or designates as the test site of a planet-busting weapon program.

Islands of democracy or exotic organizations are entirely possible; even entire democratic planets could fit into a feudal empire as long a they are loyal and pull their own weight.

- what caused the occurence in medieval feudalism in Earth's history (note that before there was Democracy(Greece) and Republic(Rome) then it switched to feudalism (medieval europe) and then is switched to democracy/republic again (modern times))? It was not a linear evolution of the government types (more like a jojo ), it seems something strange/unusual caused it. Maybe the scientific/whatever regress after the fall of the Roman Empire?

The feudal countries like England, France, and Germany didn't go from democracy to feudal. The feudal system arose out the traditional tribal cultures and power bases that expanded to fill to power vacuum left after the fall of the Roman Empire. In fact the city state of Athens went from a system of hereditary rule to one led by tyrants, at that time a tyrant was someone who ruled by popular support rather than divine right, and then on to system of democracy. Also interesting enough wealthy Athenians were required to provide ships in time of war. It’s hard to imagine now a days that if the America went to war that Apple and Google would have to donate an aircraft carrier or two to the war effort but that's would it would be like today under the Athenian system.

I could see a system in the future loosely based on the Shoguns of Japan.

The expansion into space has been taken over by a loose association of power blocs. These power blocs are made up of former nations, organizations, corporations, and wealth families each looking to stake their claim on new frontier with their own vision for it. Power blocs have their own fleets and armies and are responsible for governing their territories while pledging support to the central authority that is earth.

Territories all center around one or more strategic assets they might be a worm hole, a habitable planet, or an asteroid field rich in minerals and extend as far as the power bloc can effectively control. Colonies tend to be small with populations no bigger than an earth city, as such the vast amount of humanity still live on earth and it’s thanks to this and historic allegiance the emperor has effective control. However they have to carefully manage the power blocs and establish their own power base or be reduced to a mere figure head.