It is very likely that is the cause. Everything in the API with Groundspeak is rate limited, meaning one can only do X number of request per day with certain methods.
Those limits are extremely low for none premium members. For example I think one can only fetch information about 3 geocaches, or if they did increase it a small amount maybe.
We should probably warn our users about this, in a more clear way. Today it's mentioned on the Membership page, that some features requires premium. Building GPX files is mentioned there for example.