Yes, I'm running mods. The mods aren't causing an issue with the game itself, I believe it's the workshop. Again, it's re-downloading all 21 GB of workshop mods, even though they are already up-to-date and in the correct folder.

This may be a Steam/workshop issue, but I thought I'd check here first in case anyone has any info.

It seems I've fixed the problem by disabling all mods. I previously only had CBA running, looks like it may have caused a problem with that particular mission.

Sorry to waste your time on this, in the future I'll explore all other options first.