Error while setting Variable of type String with value of GUID

Using SharePoint Designer 2010 I've made a workflow that contains of a couple of steps:

First I'm setting a variable called GUID (type String) to the value of a List item from List1.

Secondly, I'm pausing the workflow for one minute, so that another workflow can finish it's task of updating List2.

After that, I'm trying to set a variable called DoGUID (type String) to the GUID value of a List item from List2, based on the GUID variable.

This last step is causing an error in the Workflow and produces the generic error message below. Logging shows me that the GUID variable has been set correctly, and that all steps before the error have been executed without issues.

The workflow could not update the item, possibly because one or more columns for the item require a different type of information.

The strange thing is that since this is part of a looped workflow, and I'm executing it 3 times at the same moment, one of the three executions always goes right. Sometimes it's the first one, sometimes it's the last one, but it's always 1 out of 3 executions of the workflow that finishes without problems.

Does anyone have any idea what can cause this? If needed I can send you a screenshot or anything from the original workflow.