Hi there,
same Problem with 9.400-9! We are publishing a SharePoint site via WAF, in previous versions we did not get this error. Seems to occur more often on Firefox than on IE users.
2016:03:31-10:11:13 XXXXXX reverseproxy: [Thu Mar 31 10:11:13.428843 2016] [session_crypto:error] [pid 6387:tid 4055317360] (100006)Error string not specified yet: [client IP50465] AH01842: decrypt session failed, wrong passphrase?, referer: www.HOSTNAME/.../start.aspx
2016:03:31-10:11:13 XXXXXX reverseproxy: [Thu Mar 31 10:11:13.429010 2016] [session_server:error] [pid 6387:tid 4055317360] (100006)Error string not specified yet: [client IP50465] AH01817: error while decoding the session, session not loaded: /_layouts/15/mquery.js, referer: www.HOSTNAME/.../start.aspx
2016:03:31-10:11:13 XXXXXX reverseproxy: [Thu Mar 31 10:11:13.429091 2016] [session_crypto:error] [pid 6387:tid 4080495472] (100006)Error string not specified yet: [client IP50467] AH01842: decrypt session failed, wrong passphrase?, referer: www.HOSTNAME/.../start.aspx
2016:03:31-10:11:13 XXXXXX reverseproxy: [Thu Mar 31 10:11:13.429114 2016] [session_server:error] [pid 6387:tid 4080495472] (100006)Error string not specified yet: [client IP50467] AH01817: error while decoding the session, session not loaded: /_layouts/15/1031/images/formatmap16x16.png, referer: www.HOSTNAME/.../start.aspx
2016:03:31-10:11:13 XXXXXX reverseproxy: [Thu Mar 31 10:11:13.429283 2016] [session:error] [pid 6387:tid 4055317360] (2)No such file or directory: [client IP50465] AH01822: error while saving the session, session not saved: /_layouts/15/mquery.js, referer: www.HOSTNAME/.../start.aspx
Christian