I've tried the same actions in "Do until" loop, it had 19 iterations, the flow ran several times and I couldn't reproduce the issue.
Could you try adding of a delay before the "Update SharePoint Group," I'd say 30 seconds for beginning? Perhaps there are too many requests and they are too frequent and that leads to the error. Also, try the delay with another your flow where you check whether a user is a member of SharePoint group.
Also, I would ask you to open a support ticket - send a message to supprot@plumsail.com and attach screenshots with configuration of both your flows (all actions with opened properties) and with input and output of failed actions.