Don't preview it before merging.
And you may have to rebuild the template document. Seems like once these fail from a preview, they always fail, even without the preview.
This is a known issue? OK so I try to avoid previewing it in future.
So, rebuild means from scratch or is it enough to just delete all data fields and reinsert these?
Thank you for the quick answers!
It works now.
I had to delete all affected fields and insert them again.
I find it usually works to make a new balnk file the correct size, then copy, paste in place in the new file which is faster and less likely to make a mistake.
This is definitely a known issue with preview and multiple records per page merges, but I don't recall anyone mentioning it for single record per page. If you absolutely can't live without preview, it seems to be OK to preview, then hit Undo to return to the previous screen.