The "source" of the problem is in the error message: the format string. Without knowing the format string, this is impossible to answer. Some field must have a custom format that has some content in the format string that is invalid. I suggest examining the formatting for the field(s) when the error appears. RIght-click on the field and select Format, and look for a custom format string for a number or date. Rarely, but it is possible you have a corrupted .twb file (which is basically an XML file) causing an invalid format string.
If you can post a sample packaged workbook, we can probably help.
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Do you have any worksheets named with the % symbol in use?
If so, try to remove the symbol and confirm if the behavior reproduces. There was a somewhat similar issue resolved in https://www.tableau.com/support/releases/8.1.7 , but I've also came across an issue where a worksheet named with a % symbol was causing this same error when selecting a data point, as you described. Once removed, the error did not reproduce. It is unclear as of now whether this is specific to a certain data source.
Hope that helps!