So, i had this exact same problem (Affinity Publisher+PDF export) and seemed to track down the problem and find a solution.
So, to brief it, the error message1 would come up whenever i tried to export this APub file (w/ 30+ pages, images of many sizes; text titles and subtitles, up to 5 different fonts, and interactive hyperlinked elements) into a PDF.
Initially i thought it was the size (i thought like: "really? ... can't be" - a i have a pretty reasonable rig). I tried almost all PDF export methods available in APub, and the only one that worked was the rasterize everything... which is a bit 'yucky' ...
So, as soon as i got a lil' time i went to track the problem down: I started exporting sections (pages 1-10, then 1-15, then 1-20, etc.) until i found which page was the problem, and then to make sure, i tried to export the section after it, and then that page alone. It was only one page.
So, in my case, it had nothing to do with Fonts; as i happened to find, there were 2 picture frames with very long (height wise) images inside; i rasterized those 2 picture frames and voilá, it worked. [update: It had to do with only one of the images, regular .JPG file, less than 1000 KB; see end of the post for JPEG detailed info]
It shouldn't happen, i know, and would be great if Serif/Affinity team could solve these almost random issues; but i hope it helps.
If your case is different, my 2 cents' advice is to try to track down what page(s) is the trouble-maker, and see what could be messing with the export.
1 - "An error occurred while trying to export to: (...)"
PROBLEMATIC JPEG INFO:
Format : JPEG
Width : 844 pixels
Height : 1 804 pixels
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Stream size : 976 KiB (100%)
ColorSpace_ICC : CMYK