When the fan speed is set to level II on the STR the fan enable turns off on the 121 fc controller
121 FC Firmware 1.26-11
Some units of the STR106 and STR107 may have compatibility issues with the TAC Xenta 121-FC unit, concerning the manual Fan speed wheel function.
Upgrade the 121-fc to 1.26-14 or above.
Here is the link with the upgrade files: Ecobuildings