I had the same problem when impulse supposedly updated my sins from 1.14 to 1.15.

the only response i got was "the update likely failed" whatever that means.

all i know is that when the newly updated 1.15 (1.15.48 from the old 1.15.47) that my maps all of a sudddenly came back.  Ive asked for a change log from 1.15.47 to 1.15.48 but no one has pointed me to one yet so i cant tell what they actually fixed.  (if i had to guess it was just a small perl script that opened each of the .galaxy files and changed the Version number from 2 to 3)

In short you need maps that are version 3.  All of the "old maps" are version 2.  If your looking to play a certian map with friend(s) you can have them send you the .galaxy file for that map (verify its v3 by opening in a txt editor) and you'll be able to play that single map.