So, I believe I've just resolved this same issue. It was quite simple in the end - But, took a while to narrow down without access to server logs on CPANEL.
I've recently setup my own instance of OSTicket and had been wanting to enable 2FA. I had installed the 2FA Plugin, the most recent one (Using the PHAR file) and went to setup the 2FA Plugin and instead of the QRCode and text loading up with the instructions - like in the setup video at 87 seconds - It just loads up to a blank white square. An exception as your error above suggests. (Screenshot below)
So, after extracting the PHAR for the plugin - I found the Phar uses the Github Sonata Project - Google Authenticator. The function that line 16 references above is, $auth2FA->getQRCode($thisstaff) - Which is the OTP QR Generator.
After tracing further, I found that the file being called was GoogleQrUrl.php - The generate function was being called. This generate function checks for any ':' . So, this is where I found the source of my problems. My OSTicket installation was still titled the default which of course means it has the two '::' in it's Title/Site Name.
if ('' === $issuer || false !== strpos($issuer, ':')) {
throw RuntimeException::InvalidIssuer($issuer);
}
Once I updated my Sitename and removed this - The Authenticate Plugin worked!
It would be good if there was perhaps a solution that could be implemented - Such as on the currently blank configuration page for the plugin - if a warning or message could shown if the site name isn't compatible? I could spend some time working out how to do this - But, have work soon - Maybe another time.