Correction on my last point.
I found MAX_RENUM_DELAY_MS in libsigrok (looked in the wrong place first). Increased it by a factor of 10 - now it just waits 30 seconds instead of 3 for the device to reset. Everything else is the same.
Is it officially time to give up on that particular device? SW works for uwe_ on a (most likely much newer) USBee SX. SW doesn't work with my (most likely much older) USBee SX on two different PCs => SW doesn't work with that particular USBee SX.
Oh well... was worth trying.