I had this issue with some .xls excel spreadsheets.
After selecting them and applying changes in workbench, the spreadsheet tab dropdown box would be blank so I was unable to select a tab and therefore unable to do any further work with that sheet.
I fixed the issue by doign the following:
At that point I was able to use the file with workbench with no issues.
I did not have the problem with all my .xls file just some. On one instance Excel did fix some error when reopening the .xlsm file. In my case it was important that I kept hte original file as a .xls as it is likned to hundreds of other spreadsheets.
I don't need any help with this issue. I am writing this in case someone else needs help with the same issue.