Blockly script doing nothing

[TinusExcentricus]

Trying to control ventilation via a dimmer (device WTW) based on CO2 values (device MS1 CO2). The blockly script gets triggered every minute according to log, but nothing seems to be executed. Global user variables (WTWspeed & WTWtarget) are not updated, nothing is written to log, devices doesn't get set.

EventSystem is enabled, so is dzVents.
Manual operation of WTW dimmer via slider or event works fine.

Am I overlooking something that causes the instructions of the script not to work?

[TinusExcentricus]

Simple test script to write CO2 value directly to log shows that it's empty.
All other values of the sensor do come throug. See images.