Cannot log in to one of my characters

It could also be some network trouble on your route between you and the servers.

Next time you're having difficulties run a WinMTR and keep trying to ensure that the attempts get logged by the WinMTR. Perhaps then we can see if there are any network specific things that pop up in the trace.

Instructions for WinMTR can be found linked in the Consolidated Technical sticky post. You'll probably want to trace to the host: us.login.pathofexile.com

Good luck.