Hi, and welcome!
However my account is playing up. I can't send any messages to any users! All messages go into my outbox
It sounds like you did send them. Don't worry, the messages are sent, but they stay in the outbox until the recipient opens them and reads them.
Secondly, I can't add a signature or a avatar to my account
I believe you need something like at least 5 posts before you can do this. Same with filling in your profile and signature.
Lastly, I get randomly signed out when submitting a reply to a post
There is a timer that logs you out automatically if you take too long to reply. Your account just timed out. If you are writing a very long message, copy it to the clipboard before you hit submit, just in case you get logged out.
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