Cannot add deploy key to project, no error message

I have a private project on gitlab.com and I tried to add a SSH public key as a read-only deploy key at the project level, but after clicking “Add key” the form clears and the tabs show the following:

 Enabled deploy keys 0
Privately accessible deploy keys 0
Publicly accessible deploy keys 15 

And none of the 15 publicly accessible deploy keys is mine, see What are these deploy keys?.

Am I doing anything wrong?

For the record, I was able to add other key to other project. Perhaps it’s specific to a project, or a certain key.