@Bernd Oberknapp noted the following:

The eUsage app has some issues handling Institution_IDs in COUNTER R5 reports headers. Uploading a CSV file will fail unless namespace and value are separated by '=', e.g. ISNI=1234123412341234, but the specification requires a ':' instead. For example convert the COUNTER Sample-PR to CSV format and upload it, it will fail with "Unexpected value 'ISNI:1234123412341234'". Proprietary Institution_IDs will fail unless they start with Proprietary=, but in tabular reports Proprietary must be omitted, e.g. "test:123456" would be a valid proprietary Institution_ID. The rule is that values with a non-standard namespace (e.g. 'test') are proprietary identifiers.

Also a proprietary Publisher_ID is falsely expected to start with 'Proprietary=', otherwise they are not recognized.