It must have changed, because the old Gpg4win 3.1.5 version works.
Nothing had changed on my computer. I was able to sign emails a few minutes earlier. And when I wanted to answer the next email two minutes later, it didn’t work anymore!
This seems to be a defect caused because GpgOL cannot create some temporary files, it wants to use in some settings. In most setups, it seems to work fine.