I have/had the same issue with a client server. The server was upgraded from 10.7 -> 10.8 ->10.10 Maybe that was it. I don't know.
That calendar uses the apache load balancer. The issue was that apache can not start with the load balancer loaded. The load balancer can not start because it's files in the /var/run/ directory already exits. So when you cold boot, calendar work the first time. But if you try to restart the calendar, it can't and it can't run apache for the websites etc either. What a pain! It will try to restart every 10 seconds. When it tries to do this, you see the myserver.domain.tld certadmin check the certificate in the system log. (I think that is OK, it is just verifying it)
So this is what I did. I build a server from scratch in my office and compared the /Library/Server/Web/Config/apache2/ between the two servers. I did not replace the bad one as the certificates are different.
This is what I found.
servermgr_web_apache2_config.plist loads httpd_webdavsharing.conf with com.apple.webapp.webdavsharing in the upgraded version. In the new version it does not. It also createRedirectToSSLVirtualHost with the bad version. We no longer need that.
I also noted that my migrated version has workers.properties files . I renamed them. I don't think that did much.
Finally I had to 'sudo reboot' the whole server to clear out the /var/run/ directory of the old slotmem-shm-p81db1a97--235335757_balancer files.
So
Backup servermgr_web_apache2_config.plist
mv Backup servermgr_web_apache2_config.plist Backup servermgr_web_apache2_config.plist_myBackup
This is the output of the diff statement. I hope that helps you. If you make a mistake, the system log will show you that it can not load servermgr_web_apache2_config.plist (and a whole lot more) so that is helpful.
Start with changing line 8 and 10. I did that last, and that might just do it. On the old upgraded (bad) server these pointed to the old directories.
< Means lines in the good file (from the new server)
> Means lines in the bad file so edits these lines. (from the upgraded server)
8c8 mean line 8 changes 8 chars
======
diff /Volumes/GystMedia/apache2/servermgr_web_apache2_config.plist /Users/wgnb/Downloads/apache2/servermgr_web_apache2_config.plist
8c8
< <string>/Library/Server/Web/Config/apache2/sites_disabled</string>
---
> <string>/etc/apache2/sites_disabled</string>
10c10
< <string>/Library/Server/Web/Config/apache2/sites</string>
---
> <string>/etc/apache2/sites</string>
12c12,14
< <array/>
---
> <array>
> <string>com.apple.webapp.php</string>
> </array>
[Show a differences in certificates here... don't touch]
82a85,119
> <dict>
> <key>displayName</key>
> <string></string>
> <key>includeFiles</key>
> <array>
> <string>/Library/Server/Web/Config/apache2/httpd_webdavsharing.conf</string>
> </array>
> <key>launchKeys</key>
> <array/>
> <key>name</key>
> <string>com.apple.webapp.webdavsharing</string>
> <key>preflightCommand</key>
> <string></string>
> <key>proxies</key>
> <dict/>
> <key>requiredByWebAppNames</key>
> <array>
> <string>com.apple.webapp.webdavsharing</string>
> </array>
> <key>requiredModuleNames</key>
> <array>
> <string>rewrite_module</string>
> <string>bonjour_module</string>
> </array>
> <key>requiredWebAppNames</key>
> <array/>
> <key>requiresSSL</key>
> <false/>
> <key>sslPolicy</key>
> <integer>0</integer>
> <key>startCommand</key>
> <string></string>
> <key>stopCommand</key>
> <string></string>
> </dict>
152a190,224
> <dict>
> <key>displayName</key>
> <string></string>
> <key>includeFiles</key>
> <array>
> <string>/Library/Server/Web/Config/apache2/httpd_webdavsharing.conf</string>
> </array>
> <key>launchKeys</key>
> <array/>
> <key>name</key>
> <string>com.apple.webapp.webdavsharing</string>
> <key>preflightCommand</key>
> <string></string>
> <key>proxies</key>
> <dict/>
> <key>requiredByWebAppNames</key>
> <array>
> <string>com.apple.webapp.webdavsharing</string>
> </array>
> <key>requiredModuleNames</key>
> <array>
> <string>rewrite_module</string>
> <string>bonjour_module</string>
> </array>
> <key>requiredWebAppNames</key>
> <array/>
> <key>requiresSSL</key>
> <false/>
> <key>sslPolicy</key>
> <integer>0</integer>
> <key>startCommand</key>
> <string></string>
> <key>stopCommand</key>
> <string></string>
> </dict>
155a228,229
> <key>createRedirectToSSLVirtualHost</key>
> <true/>