There is references to classes which do not exist anymore

[fayne1337]

In the file class/subscription.php are still references to prefixed classes (e.g. WPUF_User_Subscription, WPUF_Form) which do not exist anymore.

As of version 4.0.0 they seem to have been moved into namespaces, e.g.:
WPUF_User_Subscription => WeDevs\Wpuf\User_Subscription

[saiftheboss7]

CC: @sapayth

[sapayth]

some classes are still there for backwards compatibility. For example, the user updated the free plugin, but the pro version is not updated, site will crush if the classes are not there.