Thank you for your replies.
Unfortunately, I cannot compare with earlier firmware versions because I installed PicoStation directly with RC5.
Regarding the installation, the QSB was soldered under a microscope and every joint was carefully inspected. I also checked for solder bridges.
One more observation: the problem is very random. Yesterday I tested many different games for about 2 hours and it did not happen even once. On other occasions it may happen twice within 30 minutes.
Once PicoStation boots successfully, everything is completely stable:
no gameplay crashes,
no FMV issues,
CTR replay ran for over 1 hour,
GT2 replay also ran for over 1 hour,
many different games work correctly.
The issue only seems to affect the boot process.