After upgrading to 6.4, when opening certain workbooks, specific columns now display `<error>`. Upon further investigation, the only impacted columns contain conditional IF() functions that reference data in previous sheets.
The cause of this error is still being investigated. There is a current workaround that can be used to find relief while the permanent solution is discovered.
At this time, elect to "keep" the sheets that feed data either directly or indirectly into your IF() function should resolve the `<error>`.