This is git master at commit 3be142a.

I see on console warnings like this:

(gnome-calendar:16762): Gtk-WARNING ** Failed to set text '<b>xxxxxxx xxxx xxxxxx xxxxxxx</b>
xx/xx/xxxx, xx:xx - xx:xx

At "xxxxx - xxxxxxxxx - xxx" <room@example.com>\; "xxx xxxx - x - xxx" <room2@example.com>

xxx xxxxxxxx xx xx xxx xxxx xxxxx, xxxxx

xxxxx://xxxxxxxxxxxxxxxxxxxx/xxxx/xxxxxxxxxxxxxxxxxxxxxx' from markup due to error parsing markup: Error on line 4 char 53: 'room@example.com' is not a valid name

You might want to escape markup letters first, before passing them to that place, like with g_markup_printf_escaped(), I suppose.