Unable to transfer group to another top-level group

I am trying to reorganize the project structure, in order to have everything under the same top-level group (at the moment we have 4 main top-level groups). I am the owner of all projects/groups, hence I should have enough permissions to perform this operation. I have managed to transfer one group, but now I keep getting the following message (quite vague):

What am I missing? Unfortunately, the message is too generic.