Ok I've finally got this fixed.

What happened:
1) My NAS100 called USTEC and I have misspelled it for UCTEC
2) I've seen that in the log and fixed
3) Nevertheless the message got stuck in the Zorro-MQ4 message queue and the expert tried to process CMD_ASSET for UCTEC endlessly

This caused both issues - blocking MT4 UI and other threads by the endless while loop in Zorro.mq4 and not updating prices in Zorro itself, since CMD_ASSET was sent for UCTEC only and not for other assets.

A workaround: remove the pipe file for IPC between Zorro and the EA - C:\Users\Admini~1\AppData\Local\Temp\mt4sock*.sys

Suggested fix - that file should be automatically removed when both EA and Zorro broker connection are not in use.