Have you tried using a VPN to get "around" your ISP restriction?
I have this issue with my ISP, can't d/l a thing from DLS/CM. Run the VPN b4 I start Trainz, then no issue as it gets me around the IP restriction.
Maybe try with one.
I cannot download content from the DLS or even see preview pictures unless I have my VPN running first. If I don't, I see the same errors that you are getting. I can still see all the listings etc.
This only started occurring for me when I switched ISP,s. a few months back...