New users have to reset passwords otherwise it says wrong password

hi, i’m using wordpress on a multisite install and just recently this started to happen. any new user that joins and trys to login for the first time gets ‘invalid password’. the only way to get in is reset the password.
old users work fine, but it’s only new users that need to reset. after reset it’s fine.
did all the usual deactivating plugins but nothing.
am i missing something?
thanks