Finally took the plunge yesterday and started registering for my own PTA with HMRC, so that I can start to do my own tax return. Encountered a problem when it threw up an MCI error message and told me I had to ring HMRC, which I did this morning.
First person didn't really have a clue and told me to try again later, which obviously didn't help. Tried again and was told to ring Online helpdesk. They promptly told me it wasn't an IT issue, but was something wrong with my profile.
Rang the original Income Tax helpline and they put me forward to the Self Assessment section , spoke to a helpful young lady (yes they do have some at HMRC) who took away the offending signal on my SA record. Should hopefully mean that within 24 hours, I can access my PTA properly. I'll try and update once I log in to tell if it has worked.
So if you or any of your clients get an MCI error message, it seems to be to do with some flag or other signal on the SA record.
EDIT - The only other thing I could find on Google relating to this error message is an old thread on Reddit that refers to a "manual correspondence indicator"