At first it does seem to be a theme issue, since avatars show correctly with Twenty Eleven theme, although after running through the code and making some tests I can say the following:
On our actual Performs template the html code generated on the post for the default avatar is http://0.gravatar.com/avatar/a4a821503581340b2193963f23105f17?s=52&d=52&r=G, where both s and d parameters have value 52. This URL does not link to any image. Funnily enough the previous URL written with "&" (as written in html code) instead of "&" does link OK to the image (adding
amp; just after each of &), and also if we do not fill in d value for the first URL case it also links OK without using "&" http://0.gravatar.com/avatar/a4a821503581340b2193963f23105f17?s=52&d=&r=G. In quite a strange way this only happens with default avatar, since my personal avatar is linking OK using the first structure http://1.gravatar.com/avatar/925a3648f52be555c917d612e58a97b1?s=52&d=52&r=G, which leaves me quite astonished. !!??
The only relevant difference when using Twenty Eleven template would be the html code generated which for default avatar is http://0.gravatar.com/avatar/a4a821503581340b2193963f23105f17?s=68&d=&r=G. In this case d has no value, and just for testing matters if we were to assign d a value as in Peforms template, obviously it doesn't link
Conclusion : Using d value in URL in the case of default avatar (any other personal avatar works OK) produces an error, only that this error only occurs in the case of using plain text ("&" works OK).
Which would be the best way to encounter this problem in your opinion? Even though the template php code could be changed to disable d value assignment, why is gravatar site correctly accepting exactly the same URL, but only if using "&" ? Doesn't this seem more like a text-code misinterpretation issue from gravatar service? In any case, in which php file should I look for the avatar code generation?