Just noticed that in case munin.conf has exactly one node configured, for example:
[host1]
address 127.0.0.1
use_node_name yes
And the hostname does not match the one in munin-node, then munin-html blows up with:
HTML::Template::param() : attempt to set parameter 'groups' with a scalar - parameter is not a TMPL_VAR! at /usr/share/perl5/Munin/Master/HTMLOld.pm line 334
If the hostname does match, then this error is not raised.
However, this does not happen in case there is more than 1 node in munin.conf (in that case munin-html behaves properly).