I'm late to the party. I have the same issue exporting a previously Indesign project file.
For a school newspaper, we used Adobe Indesign until now. We now decided to give Affinity Publisher a try. I played around with V1 some month ago but never worked on a real project with V1. I noticed the update to V2 and installed it. I opened the last Indesign project and did some changes for the new edition of our newspaper. When trying to export the whole project that contains of 36 pages, i got the same error from above. Exporting from page 2 to 36 worked. Deleting page 1 and exporting the remaining project worked as well.
Copy the content from page 1, deleting page 1, creating a new page and paste the content did not work.
But this trick did the job for me. I deleted page 1 and created a new empty page 1. I opend the Indesign file with Affinity Publisher V1, copied the contend of page 1, switched over to V2 and pasted the content of page 1. Now the export in V2 worked for me.
I don't know exactly what's causing this error. I'm attaching 4 log files. The pdflib_paste-page1-from-V1-worked.log one was from my successfully copy/paste attempt from V1 to V2.
pdflib_single_page_1-error.log pdflib_default-first-attempt_error.log pdflib_new.log pdflib_paste-page1-from-V1-worked.log