You error message doesn't say it is an Invalid block header issue. (or is that there?)
Try an export from the database and see you get the invalid block issue. If it is there then start thinking about restoring the cube from backup.
Thanks for your ready answer.
The log does not mention an invalid block issue, but when I ran an alter database repair, it did log 53 IBH so in fact there were invalid block headers.
Doing a full restore will be somewhat costly in terms of data loss (as the backup failed a couple of times due to this same error) and we will only do that as a last resort.
Thanks for pointing out the support doc, I had not found it before when searching.
I ended up solving the issue a bit differently. Because I identified the specific data slice where the IBHs were present (a specific Year / Scenario), and because I had the data for this specific slice on our datawarehouse ready to be loaded, I just cleared both the data and the blocks for this specific slice and and reloaded everything.
Running an alter database validate now returns no IBH found.