Please enter a valid OID value (e.g. 1.3.6.1.2.1.1.3.0)

Currently PRTG's Custom SNMP and SNMP Trap Receiver sensors accept only OIDs which comply to RFC standards.

However, as of PRTG version 13.2 [Stable] OID handling will be less restrict, plus there will be an option to disable the RFC check altogether, so you can enter any OID value:

Improved [Sensors]: Added support for non-compliant OIDs for sensor types: SNMP Custom, SNMP Custom String, and SNMP Trap Receiver. New feature: In order to enter an OID into the "OID Value" field that does not begin with 1.0., 1.1., 1.2., or 1.3., add the string norfccheck: at the beginning of your OID.

A release of this version is planned for next week.