The given key was not present in the dictionary.Let me show you how to recreate and how to spot that this is infact an issue within CRM as opposed to your code.
In our CRM we have a concept of an Attendee which is how we link people to a meeting. We also have the concept of a Target entity which is a parent of an Attendee. The lookup to Target on Attendee is not mandatory as some attendees are just regular people (i.e. not linked to a "Target"). When an Attendee is linked to a meeting I want to check a target has been set and and if it is do some "stuff" based on Target information. So I create a workflow with just this check in it:
For now I am not going to add anything else as this is all I need to highlight the bug. Next, as with a lot of systems, we have bad data and not all Targets have a name. For example:
I run my workflow in its current state against this record and I get an error
Downloading the log file gives you a trace like this:
Starting sync workflow 'CG - workflow with bug', Id: e717769d-8c1e-e611-80f4-5065f38aa981
Entering ConditionStep1_step: Check Attendee.Target contains data
Sync workflow 'CG - workflow with bug' terminated with error 'The given key was not present in the dictionary.'
I'm not sure what the CRM workflows code is doing under the hood here, but it must be trying to reference the name field in code. If you set a value for Name on the Target the problem goes away.
Pro tip: Put comments in your workflows! They are included as part of the trace