Welcome to the forum!
The ellipse shape uses the Pattern Along Path LPE, and unfortunately, the LPEs don't work very well on closed paths, with corner/cusp nodes.
You could try making the nodes all smooth. Or you could try not closing the path completely. Or you could try using the PAP extension. It's not nearly as convenient, but I don't think it has this problem.
It seems like I've heard that this either already has been fixed or will be fixed by the next major version. But don't count on my memory, haha! I'm sure this must be reported as a bug already, so you could search Launchpad, to find out the progress.
https://launchpad.net/inkscape