Hey @Bence Domonkos5CE7, Welcome to our Adobe Express community!!
This sounds frustrating, and it shouldn't be happening. Did you try refreshing all the social media platform tokens in Adobe Express scheduler? Sometimes, refreshing and reconnecting the social platforms resolve such errors. You may also want to try clearing out the browser's cache/cookies to see if that makes any difference.
You can also refer to this video- https://adobe.ly/3X6n1Vj
Do let us know about your observations!
...