Hello @larsvonhierblau and @squilib ,
thank you for reaching out to us, we are here to help.
This is a known bug that was introduced to the payment plugin at the beginning of May and we have been looking into this for quite some time now. We believed that this was caused by a change on the server-side of PayPal and are still looking into that. The refund should still be possible to be performed via the PayPal web interface, so you can do that as a temporary workaround.
Last week, my developer gave me a package to test for this issue, so if you want, you can try to use this instead of the current release and see if it resolves the issue. You can download the package from here. One other merchant tested this package already last week and informed us it resolved the issue, so until the next release this could also be a possible solution. We are looking for more people to test this fix, so please let us know if you have tried it out.
Pending your response.
Kind regards,
Joost