How many database keys can I create? / database keys not showing up

Hello,

I made around 5 extra database keys, and now every database key I add in the listing setup, can’t be used in an “add custom field” under under advanced search fields.

Is this a bug? Or are we only allowed to create a certain amount of DB keys? There are still many db-keys/search options I want to add.

No upper case or spaces were used in my db-keys.

I am using the latest directory press version.