There was an update recently that (might) caused some issues with workflows.
This is not a SharePoint update, but it affects SHarePoint.
Check if you have any of these KB's installed:
KB4457915: Security Only Update for .NET Framework 3.5, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2 for Windows Embedded 8 Standard and Windows Server 2012
KB4457919: Security and Quality Rollup for .NET Framework 3.5, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2 for Windows Embedded 8 Standard and Windows Server 2012
Microsoft is aware of this issue and patches for SharePoint 2010, 2013 and 2016 are being worked as of 9/17/2018. I will update when we have an ETA. I had confirmation from the product team on 9/18/2018 that this information and solution on this post is in the line with the future patch and it is the recommended action plan until the patch is out. If anything change, I will update the post.