User blog comment:Rappy 4187/Technical Update: June 28, 2017/@comment-24258073-20170629104601

"We will fix an issue that could prevent existing accounts with a colon (':') in their name from logging in. (Note: this character is not allowed in new account names.)"

Hmm. Reminds me of a similar bug that I have fallen victim to in the past, where I was unable to post messages on message walls, forum threads, article comments, and blog posts. Possibly a result of the period in my username, though I'm not sure whether that was the cause.