Have you solved the issue? Have you tried logging value of the variable before converting action?
Maybe the value that was provided the other morning was containing information which run the workflow into this exception?
the issue is not solved, but i was able to get around
Maybe i have a problem of localization between my sharepoint (in french) and Nintex
At the beginning i wanted to display a decimal field in read only mode in a task form
It did not work because of number format, so i use a numeric workflow variable
It has worked during about 1 year, and since december not at all
So i use now a text workflow variable to get around