Solution
1. Open your site in SharePoint Designer
2. Expand the “Workflows” folder, and the folder containing the workflow you want to fix
3. Right-click on the file that ends in “xoml.wfconfig.xml”
4. Choose “Open With” -> “SharePoint Designer (Open as XML)”
5. Look for the “RulesVersion” attribute
6. Change its value to “V1.0”
7. Save the xml file
If you modify the workflow again, you’ll have to change the “RulesVersion” attribute again before saving as an STP as changing the workflow increments this attribute value which has to be reset to “V1.0”.