Full JSTOR access was restored on October 15th.

This is still happening, sadly. Here’s the latest update from JSTOR:

Service Update – October 14, 2015

We are continuing to make progress restoring full access to JSTOR.  Users should be able to search, browse, and access most journal articles and primary sources on www.jstor.org. Access to books as well as journal content published in the most recent few years may be intermittent.  If you need help, please feel free to email us at support@jstor.org.

JSTOR is currently experiencing technical difficulties. It seems there is intermittent access. This has been happening since yesterday. The JSTOR technical support team is aware of the problem and working on it. We apologize for any inconvenience.