Ok, I figured it out.
When I selected an IP address instead of a com port, I had the same problem. So not communication issue.
I checked the ASCOM trace logs. The issue is that the focuser is reported as "moving".
I have a weird configuration, where I did:
#define AXIS4_ACCELERATION_TIME 0 // in seconds, to selected rate. PROBLEM #define AXIS4_RAPID_STOP_TIME 0 // in seconds, to stopRemoving this lines solves the issue of the focuser never reporting being done with moving, and ASCOM now works.
Is this expected behavior (acceleration and stop time should not be 0), or is this a bug?
I have desired and minimum rate for the focuser set to the same value, so this isn't an issue anyway.