p.s check the user nicename in WP, and in phpBB the username_clean: it should also match the correct value; in phpBB all lowercase and also in wp, plus something else that i do not remind now like spaces substituted by – and more characters that i do not remember).
Anyway, lowercase as nicename or username_clean value should work.
If the username_clean is not correct (lowercase username) i remember that in phpBB the user where not able to login (if the username was containing spaces or some other char) because the username_clean is used into phpBB user’s sql select and the check fail to retrieve the user’s db data. Do not know if after, phpBB changed this on later versions.
They were all in the lower case. I also checked it in the WP panel. I will look again. Thanks.