Thank you for this! It now works. I found the solution just before I found your response. I'm not sure if I'm the right person to convince predictwind of anything :-)
Opening the xml (GPX) file and find-replacing all comma's with dots solved
Now the coordinates are understood by OpenCPN. No wonder this is not mentioned anywhere. It is a weird functionality of predictwind combined with the lack of warnings in OpenCPN.
Glad it works now. I'm posting
this here if someone has the same trouble in the future.
Thanks for the help all!
PS: I see this thread was posted multiple times accidentally. To the mods: these double entries can be removed.