@djensen A space is problematic, indeed, as it prevents from posting, too. Though the error message appears to be different. Replacing the space by a hyphen still caused an error message but at least allowed posting in this case. The best solution appears to be to change the username in that case.
I have edited the information in https://www.axisandallies.org/forums/topic/32464/triplea-new-stable-release-working-with-the-forum accordingly.