Good Morning,
Just to Document the troubleshooting that went into resolving the below and to update everyone that all is well again, Please see below:
Initial investigation based around the workflows themselves where we would also get errors when approving these manually. Our workflow expert managed to resolve this issue with modifications to the workflow but it was not the cause of lazy approval failures which still continued.
According to Nintex guidelines all services relating to the issue were restarted and double checked to ensure they are working as expected but the issue still remained.
We started looking into the possibility that a change made on Wednesday to the way Incoming emails were being processed as this processing has been moved to the APP server we attempted to move this back to it's original home on WFE1 but this does not seem to be possible without performing the configuration wizard causing considerable downtime for all systems.
We have then enabled further verbose logs for nintex systems to add additional information to ULS logs on the APP server regarding the failures and once tested we found the following error on the mail job sending the approval to nintex:
RunWorkflow: Microsoft.SharePoint.SPException: <Error><CompilerError Line="-1" Column="-1" Text="Type System.CodeDom.CodeBinaryOperatorExpression is not marked as authorized in the application configuration file." /><CompilerError Line="-1" Column="-1" Text="Type System.CodeDom.CodeBinaryOperatorExpression is not marked as authorized in the application configuration file." /><CompilerError Line="0" Column="0" Text="Activity 'SwitchBranchbca2bb2a045a4bf38b2000887d50a095' validation failed: Property "Condition" has invalid value. Condition expression is invalid. The condition expression can not be null." /><CompilerError Line="0" Column="0" Text="Activity 'SwitchBranch48a1c911ba0d444c90f86ca3bba3ba1c' validation failed: Property "Condition" has invalid value. Condition expression is invalid. The condition expression can not be null." />
The above was traced back to the recent Nintex breaking patches which were removed earlier in the week: https://support.microsoft.com/en-za/help/4465015/sharepoint-workflows-stop-after-cve-2018-8421-security-update
After diving into the installed updates on the APP server I found that the 4 KB's that cause issues were still installed on the APP server and removed these then I rebooted the APP server.
KB4457026
KB4457034
KB4457045
KB4457056
Tests there after were successful so this issue is now resolved.