Found this:
"Re: Only lowercase letters allowed" https://moodle.org/mod/forum/discuss.php?d=279399 (Mauno's posts at the bottom, specifically https://tracker.moodle.org/browse/MDL-46232)
So this is an existing and known bug?
Re: Bulk delete: Empty username fails with error/moodle/usernamelowercase
by Visvanath Ratnaweera -
Number of replies: 0