Invalid version specified & (must contain numbers only)

What causes this issue?

The version number specified in the RFC call &V1& does not comply with
the specifications. It must be four digits long and must only consist
of numbers (which are stored in a character field).

System Response

The RFC was not accepted; The message was not received.

How to fix this error?

Notify the manufacturer of the communication system used. It was here
that the version number was specified incorrectly.

Error message extract from SAP system. Copyright SAP SE.