The warning is shown in the Web Verification Station, however, when clicking Complete Task, the confirmation dialog displays more warnings.
This situation more likely connected with a fact that you have hidden fields that are not displayed in the data form but have enabled validation rules.
The hidden fields are the fields that are active in the Document Definition, but not shown on the Verification (i.e., the fields with the "Show on verification" option turned off).
For example, the IBAN field on the following screenshot:
Some of these fields, that are not shown on Verification, may have rules attached to them. If the rules check for such fields is not successful, the "The document has hidden fields, some of which have errors." error will be displayed on the Verification Station (this error is not visible on Web Verification Station).
You can check what fields are causing this error by making all the active fields in the Document Definition to be shown on Verification (turn on the Show on verification option).