I get the same problem. Both when leaving and adding a group. I thought it was just a bad install of 1.26.5.9 so ignored it until I got it again on 1.26.5.10 My install is always to a brand new directory so there are no files left over from the previous install. I do reuse my personal settings file, though.
The text when leaving a group is:
And when joining a group is:
(I hand typed both of those so please pardon any typos.)
Both join and leave group actions are successful, the error seems limited to being unable to display the success messages.
A snipped from the log
I've been on all day and my log file is too big to upload. I'll relog and just drop/add a group and upload that as soon as I can.
I did notice that those two alerts were never in the notifications.xml file, at least as far back as I looked. I also checked the Linden's 3.4.0.263727 viewer and didn't find them anywhere there either so I have no idea what's up.
Hope this helps.