Thanks for your reply, Vulpix. I'll try to address all your points.
How big is your profile? 32bit miranda limit is ~430MB (when using mdbx).
My backup coverage is a bit sparse unfortunately. My profile backups are between 180 and 250 MB. In late April, a couple of backups strangely reached 500MB without Miranda crashing. Following backups have OK sizes: I think I used Database -> Compact.
Your profile should be in mdbx format.
They all are, from mid-April on. Driver names: I mixed up - mdbx is the new one, got it.
Old miranda won't be able to open mdbx profile at all.
When I said "old Miranda", I was thinking of the one right before the update that broke things for me, just a few days ago. That is, post mdbx switch already.
With this "old" Miranda, I managed to open a backup from April 24, 2018, that looks fine and is 189 MB long. I exported all the settings via Miranda menu -> Dbeditor++.
Apparently, following backup .dat files won't open or import from ("Error loading source file, exiting").
Do I have any other options? What really hurts are the conversations, some of which precious, the contacts, the several annotations I made in the info field.
EDIT: I managed to sync until mid-June. Found a working backup. Searching for working ones is really a pain. It seems like the notes (User details -> Notes) are gone. Can you suggest a way to restore them explicitly? They are important for me.