| I can't explain this behaviour but to me it looks like some of the 
| informations RQDX3 stores on the surface of the disk were corrupted 
| and thus the combination RQDX3/RD53 was in trouble. Erasing all the 
| information from the disk-surface via BIOS/DOS forced the RQDX3 to 
| completely reinitialize the disk ...

We call using a PC to scrub a DEC disk the last resort.  Since I'm not
aware of any DEC utility that allows me to provide the original
manufacturer's defect list you're living on borrowed time.  The data
may be corrupted but I've used the FE version for formatting and had
that fail - the obvious cause is that unlike a PC, DEC requires a whole
lot more fixed location sectors to be okay.  If it doesn't like lossing
some/any/all of then it will get in a snit.  The PC scrub gets rid of
any evidence the RQDX3 has recorded.  But cannot fix what is a marginal
sector or set of sectors (so it is a gamble to continue to use it I think).