The problem was due to an earlier version of Helicon Focus installed, in addition to the current Helicon Focus version. Removing the old version solved the problem.

So, I am all set.

Using Capture One Windows build 15.3.0.100 with Helicon Focus Plugin 1.0.0,

Image > Edit with ... > Stack with Helicon Focus

returns

"Helicon focus version not supported"

Is this a Capture One problem?

Thank you.