Please stop “helping” me 
I suspect it is not file corruption but a consequence of an invalid repeat structure. For comparison, this recent topic illustrates what I’m talking about.
But this is still only a guess, since we haven’t seen the whole repeat structure of this file.
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The error is probably caused by a beat deleted during work, which contained a repeating line. Not a faulty repeat structure. However, when the repetition does not work, it can never be fixed again. This is the point when I actually think about not continuing to work in Dorico, but switching to another program. Even if I delete all repeats and place just one repeat anywhere in the score, the repeat will never work again. I’m very sad. After several months of work, I have to look for new software.
Sorry to hear about your issues @sandor.csernak . Perhaps you could share the project file in which you’re having trouble so someone can take a look for you?
(Posting once in one thread is generally sufficient to get a fairly quick response, by the way; you don’t need to add comments to multiple threads.)
In the meantime, I found the solution. The problem is caused by the fact that a beat that contained a repeat signal was deleted during work. Dorico will then stop playing replays. This can be easily turned back on by turning ON
the Play repeats check box in the Library/Playback options/Repeats menu.
Unless you’ve fixed the repeats in the notation (e.g. by adding back in a missing repeat barline) you will probably find that playback of repeats is disabled again next time you close and re-open the project.
In the latest version of Dorico 4, this option is now more flexible (ie it turns itself back on automatically when you fix up your repeat structures).