Can you share the workflow history for this execution please?
tctl wf show -w <wfid> -r <runid> --of myhistory.json (and share the json)
how can activity C cause a panic when the code to do the sleep is in Activity A
When you restarted your workers their in-memory execution cache is cleared and in order to continue your exec workers need to get the full workflow history thats available so far and replay your workflow code against the history. See this forum post and this video for more info.