Do any of you know why that happens?
And what can I do?
I have not
experienced that before Capture One V7, but not yet tried it with V7 converted TIFFs. It's unlikely to be caused by the TIFFs as written, because there probably was not much if anything changed for that between versions. Of course I cannot guarantee it not playing a role either, so one needs to systematically eliminate potential factors.
Does it always happen with the same files in the sequence? If no, then the TIFF formatted files can be read as intended, and Capture One is not causing it.
Then it could be that Helicon Focus has an issue reading the files, maybe due to memory issues (large pixel sizes) or harddisk read issues. The pixel size can perhaps be eliminated as possible issue by stacking temporary smaller versions of the files. If smaller pixel sizes cause no problems then there might be a memory issue you should report for investigation by Heliconsoft.
Just some thoughts.