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?
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.ezproxy.canberra.edu.au/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
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
put your comparator in if socket
what to put in x and x2
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.
sorted
not sorted
how did you show a list? do not use a loop.