Naturally, we do not discuss details of individual users. This is indeed an unusual suspension. Also note that this is a network-wide suspension, so this is not necessarily related to this user's behavior on MathOverflow.
It's quite difficult to give an answer to your question. (At least to the part about reinstating the question ban after a positively received question.)
One thing is that the algorithm for determining question ban is not public. But you also omitted quite a lot of details in your question: What exactly was the the error message you get? At the time of the ...
You may notice that this account predates MathOverflow joining Stack Exchange network in June 2013.
At the time, all users who had account on the old MO site had to merge their old account with a new Stack Exchange account, if they wanted to keep using the site and have access to their old posts. This is another instance of such case: How to merge old ...
Yes, that means the user has been suspended*. All you need to know about the situation is in the FAQ on Meta Stack Exchange:
When an account is suspended
The account will receive a private moderator message, explaining the exact details of the suspension and a box to contact moderators.1
The account will be locked at 1 reputation.
The user ...