Zotero will convert the "." into %2E in the name of the WebDav
My login name of my webdav has a dot, and it convert it into %2E to connect to the webdav, and it will fail to connect.
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In any case, the latest Zotero beta should fix this behavior, so you can install that and let us know if it works for you, and then switch back to 5.0.55 once it comes out.