Schedule shows blank page after upgrade to 4.1.2

Setup, Configuration and Use Migration
1

When trying to access the OpenHAB schedule I get the following error in log:

[rg.apache.cxf.jaxrs.utils.JAXRSUtils] - Problem with writing the data, class java.util.ImmutableCollections$ListN, ContentType: application/json

The Schedule shows a blank page.

2

Any idea what could cause this error? I have lost my schedule due to this…