I've tried taking off tags from the device until it's able to sync again - I've worked down alphabetically removing the problem tags as I go.
But once I sync up with calibre again, the rest of the tags show up and it can't sync again, despite the problem tags being removed.
I have a lot of tags, a lot of which I use to categorise my books/fanfics, but I don't need them all on the device.
I did see the blog post
here which mentioned that you could create a user category.
I thought I'd try that to select only the tags that I need on the device, but can't figure out how to get only that to sync on the device.