Hey @Ilarion, thanks for reaching out!
Modes get disabled when a variable in a selection or page (when nothing is selected) maps to a disabled mode, but contains conflicts. This can happen when the disabled mode lives in a different file and contains published updates, but the current file hasn’t accepted the updates yet.
It’s hard to be sure what may be causing the issue without seeing your file though. Please reach out to the support team directly here: https://help.figma.com/hc/en-us/requests/new
Be sure to use your Figma account email, include a link to your file, and share it with support-share@figma.com, so we can take a closer look.