This is an interesting error given that this string doesn't appear anywhere in our code base as far as I can tell. Are you using extensions in your application?
Great find @Anke, interesting stuff 
Are all the items at index 2 numbers ? If they are what type of numbers ? If they are not, then it is possible the comparision / sort will fail.
Also there are now built-in blocks for doing such a sort:
http://ai2.appinventor.mit.edu/reference/blocks/lists.html#sort
im storing numbers of victories, tag is playerName and value is victories
i have always been used the listUtil extension and its been working great
Please provide a test project as small as possible which elicits this error and post it into this thread. Please do not provide the big project.
Taifun
test.aia (12.5 KB)
not everytime this bug is happening
and i have 5 tags and values in cloudDB
Then use the buildin block please.
I found this bug previously also, but have no idea for how to fix it 
ok, how i have to sort it? what blocks buildin
x-item1 x2-item2
in fact no procedure needed here.
ok, thx
show original list and sorted list on labels and see if it's sorted.
how did you show a list? do not use a loop.