I think there may have been a timer added if you try to add too many licenses, at least I saw some evidence of this in some of the text strings in one of the newer firmware versions. You may have to leave it on for awhile to get out of that timer (which may also deplete your trial options).
And while you are at it ... take notes and write down times and stuff. If in fact Rigol did add some counter measures (unconfirmed hypothesis AFAIK), chances are fairly good that you still have a 100% usable instrument. As alank2 suggests, just leave it powered for a while to let it time out.
Not a direct solution to your problem, but more of a hint when controlling Rigol gear through SCPI. Take both the documentation and implementation with a grain of salt. I like Rigol goodies for the very nice price/performance ratio, but in the department of consistency they are not quite there yet. Basically, don't always belief what the docs say a command should be. In case of doubt best to use the full command in upper case.
And whenever you run into a
situation where you are not sure if a command just did what you expected you can check status + logs messages.