invalid characters found

[Drachenbauer]

Hello i tried to put a request for finding a retro game,
But it was at first Impossible to send it, because the new-topic-formular says invalid characters found.
Which text-characters are invalid here?

invalid characters, that i found, (i have to somewhat describe them, because i cannot send it when directly typed them):
-the key between the ?-key and the back arrow key, when not using shift
-the non english variants of a, o and u with 2 dots on top (keys between p and + and l and #)

In my case it was one of the last 3 characters, that prevented from sending the request, i am german and use german autocorrect tool on my phone, that i wrote the request with

Why are these characters invalid?
I mean, that the first one is invalid, prevents me from using shortened versions of things like "i am"

Best regards Maxi

[Rwolf]

I'm not sure, but I can write the apostrophe.
But the umlaut characters are also blocked for me.
https://en.wikipedia.org/wiki/Umlaut_(diacritic)

It could be one has to use English-only characters.

[MrFlibble]

I noticed that the version of phpBB used by the forums here (or maybe some plugin, don't know) is incredibly picky about characters. I once spent about an hour trying to figure out what was wrong with a post that I had made in a different forum and was trying to re-post here, by copying the text. It had absolutely no unusual characters whatsoever, only plain English alphabet. I assumed that maybe this was the board's way of blocking posts with "spam words", so I went through the entire post removing what I thought could be offensive words, to no avail.

Finally, I re-typed then entire post letter-by-letter, and it was accepted without any problem.