This commit http://cgit.freedesktop.org/xkeyboard-config/commit/?id=d7a239e3a0a2af299b746bc478653e573d172699 mapped 3rd level of the AC11 key to quotedbl, but it should be ssharp to match the "Czech" layout.
This bug was originally reported at https://bugs.launchpad.net/ubuntu/+source/xkeyboard-config/+bug/953477.
wait a sec, quotebl is on the 4th level of AC11. The 3rd level is apostrophe. Which one do you want to change?
My apologies, I meant 4th level; I changed the bug title accordingly.