I am getting the same problem (Knime 4.1.0). A couple of days ago I noticed some Excel Writer nodes, in a couple of workflows, were marked as deprecated, so I deleted and replaced them with the latest Excel Writer. I then ran them OK and outputted & opened some Excel files. Yesterday I didn’t try these workflows, but today I ran them again and now, in both workflows, the Excel Writer node throws this error. I tried copying in an old deprecated Excel Writer into one of these workflows and that worked OK. Replacing the node with a fresh new version did not work, however.
I tend to be nervous about mass-updating my Knime (e.g. I am avoiding the Update prompts in the Welcome screen) as in the past I have clicked on these and then found that some stuff did no longer work (certain nodes got ‘left behind’ etc). Anyway, I will try updating to 4.1.1 and report back.