After converting a vendor specific MIB file into a Paessler oidlib with Paessler's MIB Importer Tool and using this oidlib as source for the creation of a SNMP sensor in PRTG, "Error converting OID" appears in the name of the sensor. The sensor, however, works as expected.
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".
Edit the name of the sensor in PRTG to your liking.
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.