This has happened twice now...I'm pretty sure these are the steps I followed.

This happens after play 4-5 games on SP on one map and then switching to another map...not reproduceable from a clean start.

When playing SP, regardless of map size, you have 3 slots to assign DG's...the other two slots are preloaded with 'closed'.

When you try to assign a 'closed' slot to a DG for Forces of Light (I was doing hard ai's), subsequently 'kick the player' and then re-assign, it will not allow you to enter a DG in the 2nd 'avail' slot on the Forces of Dark. When you try to convert a 'closed' on FoD, it will not work and now will not allow you enter any more DG's on either side. You can leave the lobby, you can go back up the menu, and this will stay the same (unable to assign DG's). The only way to fix is to close the game & restart.

Here are specific steps:

Assign Hard DG to slot 2
Assign Hard DG to slot 3
Assign Hard DG to slot 4 (which is default to 'close')
Kick DG in slot 4
Assign Hard DG to slot 4 again

Assign Hard DG to slot 1 in FoD
Try to assign Hard DG to Slot 2 in FoD (unassignable at this point - You select but it doesn't populate)
Assign Hard DG to slot 3
Try to assign Hard DG to slot 4 (which is default to 'close') which doesn't work

If you clear a DG on FoL which you already assigned, at this point (which will succeed), no more assignments can be made.

I know that kicking a player / re-assigning is not the right way as you should just click on the player to pick another DG, but if you do it the way I did it, it causes an issue.

Again, this occured after playing several SP games on one map and then switching to another larger map and using more than 3 DG's....