Explanation

This usually happens with SNMP values that are organized in tables (i.e. there could be more than one entity or instance to monitor, like multiple CPUs, etc). The MIB Importer tries to determine a name for the instances from the MIB. If this fails, the MIB Importer uses the table index as a means to distinguish the instances from one another. Sometimes, however, the index value is made not accessible by the manufacturer of the device (e.g. because the index in the table might change after a reboot). This results in the described "Error converting OID".

Simple Solution

After adding the sensor, edit the name of the sensor in PRTG to your liking.

Advanced Solution

Advanced users only, MIB knowledge required!

To avoid a wrong sensor name in advance, try to locate the MIB entry that represents the name value in the MIB file. In the MIB Importer change the OID that is displayed in the "group" field after the #[ to the new value (in most cases it means to exchange the last .1 with a .2 or .3 or whatever number the name value is given). You have to do this for each OID entry you want to use in PRTG. Export the oidlib to PRTGs oidlib directory and use it there to recreate the sensor.