invalid template: a-file-that-doesnt-exist.tpl, or template directory: ...
A template tried to use a template file that doesn't exist. For example:
{jrCore_include template="a-file-that-doesnt-exist.tpl"}Jamroom looks for the file in the active skin's folder, here skins/(your skin)/a-file-that-doesnt-exist.tpl, and can't find it.
This often happens after switching skins: a template copied from another skin includes files only that skin has.