Excel export error "String value too long"

There is very little information here that we can go on.
For example , there may be more information in the error log (on the system tab).

Also when you say "export a form", I presume you mean export a cube view : So have you tried opening the Cube View from the Excel add-in to see if that works?

Does it work if you try to open the Cube View from the Embedded Spreadsheet component?

Does it work exporting other Cube Views to Excel?  If so, then what are some key differences with this particular Cube View?

That would then throw some light on whether it's an Excel limitation or something else.

That error message looks more like a DevExpress error, rather than a OneStream error.
Is the cube view trying to show large amounts of text?  I believe Excel has a limitation of 32k characters in a cell.
So if you have a cube view with a ton of cell text detail , e.g. on V#Annotation, then that might blow the Excel limit.

But there is no context here , so it is difficult to answer.

Since this is more an error and you need a support level response then these sorts of questions would better be handled by opening a ticket with OneStream Support, stating the OneStream version, some context, some information of the Cube View, whether it works from the Embedded Spreadsheet, some information from the system error log etc...