had something similar happen to me last weekend at the chili event, could not get the GPX file to load in my 550, It ended up being a puzzle cache in the area.
I had the same problem loading caches into my Nuvi. The problem is with the way some logs are written using Emojis from the Apple operating system. Example can be found in cache GC3B1AQ Geo-Idols III – SuperCacherMan log by mkeiron dated 03/24/2013. The charcaters between “unsignable & TFTC” are causing the problem. The simplest solution is not to load these caches or have the log owner remove the offending characters. I have a list of caches with these characters are being used. Anyone would like this list of caches I found send me an email.
If the problem is logs, you can remove the offending logs from your GSAK database. Just right click on the log, select delete logs, and check the ones you need to remove.