Assignment error

[wurzel]

I have a rather large topic (a sorcerer's tower) that takes too long to load. It has some detailed maps of the different levels so in order to minimize the large topic I intend to split it into a number of smaller topics for the levels.
After creating the level topics I move the maps to the corresponding new topic. That works fine for all maps except one that doesn't want to be moved. It says "This topic is unavailable for assignment because it is already assigned, or the filter settings exclude it".
It is exactly the same format as the other maps have, was created at the same time with the same settings. What is the possible difference? Why can't I move it? The error message tells me nothing, I simply don't seem to understand it.