I am having a similar problem after updating the firmware from 4.30 to 4.40 in a Nuvi 755. I cannot calculate routes for locations outside the US 48CONUSA. I have tried to calculate some routes in Panama (between two locations) using the SouthAmerica 2012.40 map and I cannot do it. However, I can calculate routes within the USA using the NorthAmerica 2012.40. Before the upgrade I was able to calculate routes without any problem using different maps. I have tried many of the suggestions mentioned before (re-installing maps and different versions of the firmware) but it is not working. This is a puzzling question that I would love to understand and maybe solve.
As I said in a previous posting, I had the same problem after updating the firmware to 4.40. I contacted Garmin and they made a lot of recommendations but I could not solve the problem. I am TomTom user (I do prefer the TomTom GPS since it is more "user-friendly") but I have a Nuvi Garmin (755t) that I use when I travel to places where I cannot use the TomTom.
I spent about a week dealing intensively with this problem and doing many things without being able to solve the issue. I was convinced that something got corrupted when I did the update and the problem was to find what file (or files). After many deletions and changes I think I found the source of the problem. The original NorthAmerican maps (both the regular and the 3d) plus the JCV were corrupted. I deleted all the img files and all the JCV files and I only kept one map (the one I will be using in Panama) and it is working again. I renamed that map from GMAPSUPP.IMG to GMAPPROM.IMG and now it is working fine. I can calculate routes in Panama and do all the things that I could not do before
like creating custom routes in advance.
As I said, I am not an expert on Garmin software or hardware and I solved the problem I had by trial and error until I found the source of the problem.