As has been announced several times (most recently at
https://lists.wikimedia.org/pipermail/wikitech-l/2015-April/081559.html),
the default continuation mode for action=query requests to api.php will be
changing to be easier for new coders to use correctly.

...

The simple solution is to simply include the "rawcontinue" parameter
with your request to continue receiving the raw continuation data
(example <https://www.mediawiki.org/w/api.php?action=query&list=allpages&rawcontinue=1>).
No other code changes should be necessary.

Or you could update your code to use the simplified continuation documented
at https://www.mediawiki.org/wiki/API:Query#Continuing_queries
(example <https://www.mediawiki.org/w/api.php?action=query&list=allpages&continue=>),
which is much easier for clients to implement correctly.