Diagnosis
The field name for the single field condition does not exist in thefield catalog. The characteristic may have been removed from the databasis without the single field condition having been adjustedafterwards.
If the field name exists, it might be a referenced field. However,single field selections can be created for basic fields only, and notfor fields that reference other fields.

Procedure
Add the characteristic back into the data basis. Then you will be ableto delete the single field condition. If the name of the characteristicis empty (that is, if it contains a null string ""), then a programerror has occurred.