UMC bans map with no regards if they fit the current playercount, which leads to two possible problems:
If you set the number of previously played maps banned too high and the playercount does not change you end up with 0 maps on the mapvote.
If you set it too low and the playercount changes you end up with maps banned that are not relevant to the current playercount.

It should be changed to only ban previously played maps that are relevant to the playercount at the time the vote is triggered, and always allow a minimum of two maps not banned.