I have had the same issue, but it does not appear that others have answered. Wondering if you have received any answers independently? I have an Elgato 250 plus and an iMac 27" computer for background. I also sent a query to Elgato last evening - no response as of yet though.
"I have successfully recorded through my eyeTV 250 plus older home movies that had been recorded in several formats (VHS tapes, 8mm cassette tapes, some DVDs where I recorded some old tapes via a Panasonic HDD, and in some cases I had to use the Master DVD RAM disks as some of the original tapes were degraded. They transferred successfully as eyetv recordings.
Next I exported these to iMovie. In total there were 90 recordings that I made. Most transferred satisfactorily and were ok. But a total of 16 gave me an error message as follows (one example):
"Unable to update thumbnails for project '1996-09_ PMHS Soccer - Roberto - PMHS vs Western Wayne - September 3, 1996.rcproject'. Your disk may be full."
The disk is not full as I have some 800gb of total disk space. I do not know if it is due to a recording issue or choices made when exporting or an iMovie issue. I have not been able to find any on-line similar issues published, so I am exploring this issue with you as well as Apple in parallel. I also recall seeing another error message once referring to "quartz video card??".
