changing weight of blocks inside region fails

changing weight of blocks inside region fails, with the following error: An illegal choice has been detected. Please contact the site administrator.

this issue happens when i try to drag the blocks, even if i change the block config page to 'Show row weights' and try to enter thei weights manually.

the same error appears, when trying to set a region for a block, directly inside the main block config page. when setting the region inside the block configuration at 'Region settings' it works, without any error.