Internally the drives may still have a different name, so it could be causing an issues. Personally, I'd made sure all the files are in 1 parent folder (or as few parent folders as possible. Then I'd have moved the files and catalog to the new drive, and then used Find Missing Folder or Update Folder Location to match to the new locations.
I already tried that :-(
The update folder location was without problems. All files have been found and seem to be OK. But the initial problem then still persists.
It seems that some internal reference got messed up. Is there something like a checksum which Lighroom keeps for all the files? Maybe this got out of order at the copy of the files?
Just an additional note:
When I tried to make the panorama, it is put in the same folder on the disk, but gets a new folder in the Lightroom Folder view at the left side. When I now try to move the original files into the new Lightroom folder (which is in fact the same directory on the disk) I get "An internal error has occurred. ?:0: attempt to index field '?' (a nil value)"
How did you get the files over in the first place? If you transfered using an external drive, maybe the solution is to point (using Update/Missing) at those folders on the drive and then move them over to Lightroom after they've connected?
I shared the files on the old Mac and transfered them via network to the new Mac.
You should be able to connect them over the network as long as the catalog is on the new machine.
Now I moved the photos to a new folder and started with the Lightroom library from the old Mac and connected the files via "Find missing folders". It works now. It seems that its better to re-connect the files instead of relying on the same path.
Thanks for the help!