Thank you to all who replied with feedback and possible causes/solutions.
Honestly, I don't know. The post is not long and it doesn't contain bad words.
Without me knowing it, it's possible it might contain words that are forbidden on this platform, but I have no way to tell.
Someone else suggested it might be a random bug already occurred in the past, so I will keep trying.
If it's a specific post you can't post, it is most likely something in it, such as either banned words, like it was suggested, or more commonly tag problems, like one not closed, or in case of threads a too long title, or an attachment that's too big (2 Mb is total, not per file). For all of those bar the title, you could try to post it in an existing thread as a test, and if it works edit it out if you don't want to leave it there. And if it does work in that case, it at least reduces the options to either title problem or a strange bug affecting your account's ability to start threads.