Thank you José for the reply. I have tried the GluonFX 1.0.26 plugin before. It also produced errors. I tried it again. It did not work:

mvn gluonfx:build failed again.

Somebody on this forum suggested:

mvn gluonfx:compile

followed by:

mvn gluonfx:link.

This worked! Not sure why. (build = compile + link). It is probably a gluonFX bug. Thanks again for the great help.