Protected sheet blocks OneStream parameter dropdown refresh (Validation.Add / AddDataValidationComboBoxFromList error) — workaround needed

Hi all,

We're running into an issue with OneStream parameter dropdowns (combo box selectors) on protected Excel sheets, and I'd like to check if others have found a clean solution.

Issue:
When a OneStream parameter (combo box selector, e.g. ParamSelector_Default_...) is placed on a protected sheet, the initial value displays correctly on open/refresh. However, as soon as the user tries to select a different value from the dropdown, we get the following error:

Exception from HRESULT: 0x800A03EC
User Interface Stack Trace:
   at System.RuntimeType.ForwardCallToInvokeMember(...)
   at Microsoft.Office.Interop.Excel.Validation.Add(XlDVType Type, Object AlertStyle, Object Operator, Object Formula1, Object Formula2)
   at OneStreamExcelAddIn.XFExcelRange.AddDataValidationComboBoxFromList(IXFCommonWorkbook workbook, List`1 listItems, String defaultValueOrNull, Boolean setValue)

We've isolated this to sheet protection specifically:

• Reproducible on a brand-new file with a single new parameter - not related to any residual named range or leftover validation.
• Reproducible across multiple machines/users (a client reported the exact same behavior independently).
• The parameter cell itself is already unlocked (Format Cells > Protection > Locked unchecked) - this did not resolve it.
• Removing sheet protection entirely resolves the error immediately.

I’ve found suggestion that the issue might not be related to the locked state of the cell, but rather to Validation.Add being called while the sheet is protected at all. It seems the add-in needs to rebuild the dropdown's data validation on every value change, and that call fails outright on a protected sheet regardless of cell lock state.

Workaround we're considering (but haven't tested/validated yet):
Using Protect ... UserInterfaceOnly:=True via a VBA macro (triggered on Workbook_Open, since this protection mode isn't persisted in the file) instead of standard sheet protection, so that programmatic calls (potentially including the Excel Add-In's) can still modify the sheet while manual user edits remain blocked.

Questions:

1. Has anyone else encountered this specific error with parameter dropdowns on protected sheets?
2. Is there a supported/documented way to keep a sheet protected while still allowing OneStream to refresh parameter dropdowns, without resorting to macros ?
3. Is this a known limitation of using parameters in the Excel Add in, and if so, is there a fix planned or a recommended template design pattern to work around it (e.g., unlocking the whole parameter row/area, using a separate unprotected parameter sheet, etc.)?

OneStream platform version: 9.2.1.18605

Thanks in advance for any pointers!