Hi thank you for your review and raising your concern. It could be that you found an avif image that is not yet supported by the Avif detection algorithm. Also in case you have installed other extensions there could be a chance that they would interfere with the conversion. Alternatively a webserver is simply serving jpg, png or another format image while wrongfully adding the .avif extension.

To be sure could you please verify you can see and convert the avif image on this page: https://libre-software.net/avif-test/ ? if not try resetting the addon and try again. If this image works and you can convert it please send the image or a link to the image you are trying to convert. Than I can analyze it and if nessecary update the algorithm.