I have Adobe Flash Player 22.214.171.124, Firefox 57.0.4 and Windows 7 64bit.
My problem is that Firefox displays the Flash contents Server Error. When loading the content hangs the Flash Player. If I start the second time Firefox comes only once Server Error, then go the Flash contents.
Sounds like the error is coming from the server delivering the content, which you would need to contact the content developer for.
Can you post a screenshot and the exact URL to the site in question?
I use Win7 64 bit, Firefox 57.0.4 32 bit and Adobe Flash Player 126.96.36.199 64 bit Firefox.
According to Google translate, the error is "Flash Player loading error. Download Adobe Flash Player for the best quality or switch to the HTML5 player" Please provide correct translation if this is incorrect.
If the Google translation is correct, this is not a server error. The browser is blocking Flash Player and you need to enable it. Most browser's block Flash by default, forcing user's to enable it. To enable Flash in Firefox see Why do I have to click to activate plugins? | Firefox Help