Situation

I get the error message "Templates already exist" whenever I try to do anything involving a
Template. This includes opening an existing document based on a template.

First of all, you have to know that your Libreoffice profile has moved from $HOME/.libreoffice to $HOME/.config/libreoffice.

Solution

You just have to rename $HOME/.config/libreoffice/3/user/registrymodifications.xcu to ~/.config/libreoffice/3/user/registrymodifications.xcu.old

A new file registrymodifications.xcu will be created the next time.