Manual assign a subscription from frontend or backend

Ok thanks, so maybe i'm misconfigured something. I have 3 plans (all 3 are identical except for the price)

and these are my registration settings:

   
 

and they correctly show up in registration:

 

but if a user has no subscription at all i cannot give them one:

 

Could this be a consequence of the option "disable upgrade to this plan"? Like the system sees the "lifetime subscription" as plan and prevent me to "upgrade" to a real plan?

Subscriptions for existing users are always considered Upgrades and you have upgrades turned off for your plans.