@Thasaidon I put <p><br/></p> into an otherwise empty Sigil ePub and ran the ePubTidy plugin but I did not get any error messages. The plugin inserted into the line as expected to obtain:
<p> <br/></p>
is inserted to ensure that this gives a blank line in Sony readers.
Perhaps another part of your epub is the source of this error? Please try running the plugin on a Sigil ePub that contains only the line <p><br/></p> to check whether this still causes the issue.