Hey Chris, you probably already updated your Lion server, but I have had success changing the 2 instances of SSLv3_METHOD to TLSv1_METHOD in each file below using TextWrangler, using the following steps:
/usr/share/caldavd/lib/python/twistedcaldav/stdconfig.py and
/usr/share/caldavd/lib/python/twext/internet/ssl.py
Steps:
BACKUP YOUR SERVER. You are messing with files under the hood. You may want to export all calendars, too using 10.10 or earlier.
Turn off iCal Service
Open those 2 files in TextWrangler
Change the 2 instances of SSLv3_METHOD to TLSv1_METHOD in each file and save.
Turn on iCal Service.
Try to login using Calendar on a 10.11.x machine (I used 10.11.4). It should fail. If it doesn't you are good to go, but you should also verify a machine with 10.10.x
If it fails, turn off iCal Service again.
Change the four instances back from TLSv1_METHOD to SSLv3_METHOD. Please note in the stdconfig.py SSLMethod line there will be 2 instances of TLSv1_METHOD after the hashtag comment since you changed SSLv3_METHOD previously. It doesn't matter that you change the comment actually, but it's good to remember the options you have.
This is the line I am referring to:
"SSLMethod" : "SSLv3_METHOD", # SSLv2_METHOD, SSLv3_METHOD, SSLv23_METHOD, TLSv1_METHOD
in the stdconfig.py file.
Save the files.
Start the iCal service again.
RESTART the server.
It was at this point that I was able to get 10.11.x and earlier clients to login again into their Calendar and iCal accounts. This also worked with iOS 8 and iOS 9 Calendar app logins.