That’s likely nothing serious: It simply means that they’re using a SSL-Certificate which is issued by themself. Otherwise they would have to pay some money to “Certificate Authorities” every year without any improval to the connections security. When confirming that you want to continue anyway you could check if the certificate’s MD5-Fingerprint is “52:B6:3C:A8:97:70:C0:4C:45:65:DF:24:1D:BD:47:9B” to be extra sure. For further informations about this have a look at the Wikipedia-Articles about CAs and TLS.
This warning was added by the user, who reported the bug (I toned it down a little). It’s a problem specific to his camera model.
If the plugin worked for you in the past, then you are very likely not affected by this bug. (The plugin itself hasn’t changed much in the last year.) You can check the EXIF data after geotagging for one sample image, if you want to make extra sure, everything is alright.