Happened to me too and on my side it is a certificate problem. Server 5 seems to have changed its handling of certificates, resulting in errors in the server website protocol like these:
AH00016: Configuration Failed
[Sun Sep 20 10:26:17.198636 2015] [ssl:emerg] [pid 1897] AH02580: Init: Pass phrase incorrect for key localhost:8081:0
[Sun Sep 20 10:26:17.198806 2015] [ssl:emerg] [pid 1897] SSL Library Error: error:0D094068:asn1 encoding routines:d2i_ASN1_SET:bad tag
[Sun Sep 20 10:26:17.198818 2015] [ssl:emerg] [pid 1897] SSL Library Error: error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag
[Sun Sep 20 10:26:17.198830 2015] [ssl:emerg] [pid 1897] SSL Library Error: error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error (Type=RSA)
[Sun Sep 20 10:26:17.198842 2015] [ssl:emerg] [pid 1897] SSL Library Error: error:0D09A00D:asn1 encoding routines:d2i_PrivateKey:ASN1 lib
[Sun Sep 20 10:26:17.198848 2015] [ssl:emerg] [pid 1897] AH02311: Fatal error initialising mod_ssl, exiting. See /var/log/apache2/error_log for more information
[Sun Sep 20 10:26:17.198854 2015] [ssl:emerg] [pid 1897] AH02564: Failed to configure encrypted (?) private key localhost:8081:0, check /etc/certificates/r2d2.atoenne.de.7A40030F6DA5DACB8F0CAE37634FBAE589B351CA.key. pem
AH00016: Configuration Failed
Certificates show ok in the Server app.
The site behind 8081 was a localhost only myphpadmin. Deleting this site resumed the httpd service. Interestingly enough, another SSL secured site is working ok.
This did work before Server 5!