I Will start with the basis that this may not be a Dynamo Issue.
After having swapped out a computer and installing Fresh Revit 2018 & Dynamo. The ability to write/read to Excel no longer works. This has been attempted in Dynamo Revit / Sandbox / Studio - None work.
Excel Functions, and Revit Function as normal other wise.
Curios if anyone else may have seen this condition?
@Mike_Engel1 Do you have Microsoft Office 365 Business by chance? I recently had this problem after Office 365 updated, and @Kulkul recommended uninstalling and reinstalling Microsoft Office 365 Business - after this my Excel nodes in Dynamo worked again. This thread here also offers some other solutions: