Update: CDL has contacted ACS and we think the proxy blocking has been resolved. We are continuing to monitor this, so please contact us if you have problems getting to the articles.
We have received reports from users who are unable to access the American Chemical Society Journals website, instead getting a message: client IP is blocked because session duration longer than 480 minutes. ACS recently migrated the journal content to a new website, and several other campuses are reporting similar problems (particularly with proxy access).
If you get blocked from their site, please contact me (firstname.lastname@example.org) and include in the message whether you were on/off-campus, using wireless/proxy/VPN, and your IP address if possible. We have contacted ACS and hope to get this resolved soon.