Did you download and configure DavX5 for yourself?
If yes, you should do the following:
/e/ has DavX5 integrated into the Accounts Manager. If you go into Settings->Accounts, you can add an /e/-account. Then everything should work fine.
I do not know how things behave if you leave DavX5 installed alongside, so I would uninstall and use only the integrated version (works also with any other additional calendar, etc.)