That regular User account should be mapped to the PI Identity (or PI User/PI Group) which should be specified in the Data Security of the tag.
First, go to the PI System Management Tools (PI SMT)->Security->Mappings and Trusts. Find the problem User account in the list of PI Mappings and note the corresponding PI Identity.
Then go to Points->Point Builder -> Security tab and make sure that PI Identity has write permissions in the Data Security list. For editing tags in bulk you can use PI Tag Configurator plugin for Excel.
If you still doesn't work, you might want to take a look at PI SMT->Operation->Network Manager Statistics. Find your application in the list and then check to see if the application connects with the proper mapping and uses the proper PI Identity.
Anna - thank you! That worked like a charm.
Security settings always seemed so foreign to me, but now that I understand you can even set it per tag, that makes so much more sense.
Thanks again - you're the best!