Yeah I think this is the problem. Our 90 was only 5 months old when it happened but it was on the way home from an outback trip involving lots of dust. LR replaced the lock at next service but it since happened again, fob won't unlock but key with some gentle persuasion will. Now we're out of warranty I'll probably make more of an effort to investigate next time but I guess the best tip is to keep it clean.
EDIT: Just to clarify, pretty sure it is crud because once you do unlock it, in our case anyway, it works fine again until some random time later.

