Illegal mix of collations (ascii_general_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE)

[Soma]

Page: https://sev-online.ch/de/members/login/

User: guest

Error: 

Exception: SQLSTATE[HY000]: General error: 1267 Illegal mix of collations (ascii_general_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '=' (in /xxxx/wire/core/DatabaseQuery.php line 86)

The first and only time I get this error on this site. It's on the login/register. I'm not sure what exactly causing it, but I'm sure there's some special char getting submitted? It's hard to tell just from the vaque error message.

Any ideas how to reproduce it?

[Soma]

Just found how to reproduce it. When I enter some special chars like "éààéé" as the username...

I think I need some more validation/sanitizing for that.