I installed Girona on Ubuntu 17.10 and it never worked for me. I just updated Ubuntu 18.04 LTS and Qgis displays the following message:
Traceback (most recent call last): File "", line 1, inImportError: /usr/lib/x86_64-linux-gnu/libQt5Core.so.5: version `Qt_5.10 'not found (required by / home / user / .local / lib / python3.6 / site-packages / PyQt5 / QtCore.so)
And it does not connect to the plugin repository.