I don’t know the exact cause, but I can make a suggestion. I would try to delete your existing md+ connections and see if the error goes away. Note: you would have to recreate them again later. Try this.
Install run latest toolbox extension.
Enable advanced mode.
Online banking tools menu.
Force disconnect md+ connection.
Does a list of connections come up?
Try deleting one? Did it delete ok?
Restart MD. Same error?
Repeat until error goes or all connections have gone.