I saw this issue this past week. Here is what happened to me. I have been using the same payment type for all that time. My trading limits were up to Level 1. Now about two weeks ago I put an alt on my payment info and bought some L's for him. No problem, didn't affect me at all, I remained at Level 1 trading limits. This past weekend I put another alt on, bought some L's and it reset my trading limits to day 1 limits. So I received the "We were unable to make changes to the current invoice". When I tried to buy over the 30 USD day 1 limit on my main account.
My guess that people seeing this are making some change to their billing, changing the billing information, adding another account or something along those lines, then trying to spend more than 30USD in making a purchase and getting rendered "n00b!" like I did. Remember, folks, for 24 hours after LL decides you billing information is "new" you are basically held in a penalty box.