I had white screen in admin area (all ok in frontend) with a version of 5.2.6 (i had 5.2.5) of 16:51 UK time.
So I downloaded latest (I guess 5.2.6.4 ??? no indication), and tried upgrading a second time from 5.2.6 (of 16:51 UK) to 5.2.6 (of 19:01 UK) without copying the install folder (to avoid having to remove it again), and then got the same message "Down for Maintenance (Err 2)". I then tried adding the missing install folder, went through the same process, with upgrade process accept license and upgrade database, then deleting install folder, then going to admin still got same message "Down for Maintenance (Err 2)".
So looks like upgrading from 5.2.6 to 5.2.6 gives that message in front and admin areas.
I restored 5.2.5 folders+database from backup and all back to normal (but not security patched).
Waiting for a stable 5.2.6, it's nerving that you don't publish the last .version with the filename at very least. Keeps us making guesses.