I'm really sorry to hear stories like these still happening, keep us informed...
Being a developer myself, I could say that the '0000-00-00 00:00:00' on last activity column is something an hacker who would only go through a brute force attack on your password couldn't be able to change using only the frontend interface, apart taking advantage of a vulnerability that only he knows.
So the hypotesis could be:
[1] a faulty user(s) update/migration on the database at some point during the site lifetime by the admin(s) or
[2] a scarier one, that someone being able to access the database did what he wanted and than in order to cover his tracks resetted the timestamp column, but not in all cases the column would have been updated with the last activity in first place, depending on the action/query the malicious person did, eg: a timestamp column get updated automatically in some cases.
[3] a server crash that may have broken your record in the account table, this is quite rare but still may happen, in this case only a previous database backup could prove it.
[4] others that I ignore...
This actually is the last bet timestamp, so pure investors will have it forever say 0000-00-00 00:00:00. I have corrected the profile pages to say "No bets placed" instead.