When I discovered what made b4a halt constantly, I timed it while watching the autobackups directory.
A file was created there every 1 minute and then disappeared after a few seconds.
I assume backup failed since the last remaining autobackup file is dated the day before all this happened.
I was running Debug mode at the time, but it was no different in other modes.
Had done several compiles and installs on a tablet using b4a bridge at the time of timing.
I also had rebooted fresh in case a waiting upgrade was slowing the pc, before I discovered what it was.
Possibly it waited 10 minutes before it tried it first time.
Could the backup failure have had something to do with the icon file. I choose a new file and deleted the old one, then synced the files folder. But every time I open the project it says the old file is missing.
I have disabled AutoBackup now, thanks DonManfred.