SSRS - Export to Excel "We found a problem with some content" " Do you want us to try to recover as much as we can?"

This error occurs sometimes if you have null values, or values that export to excel as -0, or any other type of invalid formula that causes Excel to throw errors when it reads the file.

Review all fields on your SSRS report to ensure they are exporting valid values to excel that will not be accidentally translated into bad formulas.