Function generator Toellner TOE7720 - Error 5

[Swake]

Hello,

Got a programmable function generator from Toellner model TOE 7720.
On power up it shows Err. 5 on the display.

Pressing the 'CLEAR' button eliminates the message from the display and the generator starts working. The error might be related to some form of internal memory as there are buttons for 'STORE' and 'RECALL' but I don't know what it does precisely and cannot find out how and if it works.

If anyone got an idea what that error 5 might be or has a manual or schematic I would appreciate the help. Thanks in advance!