I was able to replicate this error on my old Linux machine. It appears to have been caused either by an incorrectly set Sigil Python 3 binary path or problems caused by installing Python 3 alongside Python 2.
On my old machine, the Python 3 pat was set to /usr/bin/python, which was linked to the Python 2 executable.
After changing it to /usr/bin/python3 and deleting the Python 2 path, the plugin worked fine.
If that doesn't work for you try removing the embedded BeautifulSoup4 package and install it separately with pip or pip3.
1. Open Sigil and select Preferences from the Edit menu.
2. Click Open Preferences Location. This will open the Sigil preferences folder
3. Open the plugins folder, which contains the URLChecker folder.
4. Open the URLChecker folder and delete the BS4 folder.
5. Install the Python 2/3 versions of BeautifulSoup:
Code:
sudo pip install beautifulsoup4
Code:
sudo pip3 install beautifulsoup4
Then re-test the plugin with either the Python 2 or the Python 3 Sigil interpreter path empty. One of them should definitely work.
P.S. On my old machine I got an IncompleteError when I tried to install beautifulsoup4. If this happens to you, too, you'll have to re-install/update pip/pip3.