UTM 9.5 WAF: slotmem_create failed

Have never seen this before, but at one customer, I upgraded for UTm 9.5 4 days ago, suddenly all servers had "internal error" and a yellow exclamation mark in WAF settings. Live log was full of this:

017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.000810 2017] [proxy_protocol:notice] [pid 21492:tid 4152121024] ProxyProtocol: disabled on 127.0.0.1:4080
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.001023 2017] [security2:notice] [pid 21492:tid 4152121024] ModSecurity for Apache/2.7.4 (http://www.modsecurity.org/) configured.
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.001035 2017] [security2:notice] [pid 21492:tid 4152121024] ModSecurity: APR compiled version="1.5.1"; loaded version="1.5.1"
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.001051 2017] [security2:notice] [pid 21492:tid 4152121024] ModSecurity: PCRE compiled version="7.8 "; loaded version="7.8 2008-09-05"
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.001060 2017] [security2:notice] [pid 21492:tid 4152121024] ModSecurity: LIBXML compiled version="2.7.6"
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.207757 2017] [slotmem_shm:error] [pid 21497:tid 4152121024] AH02599: existing shared memory for /var/run/apache2/slotmem-shm-p98831e12.shm could not be used (failed size check)
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.207917 2017] [proxy_balancer:emerg] [pid 21497:tid 4152121024] (22)Invalid argument: AH01179: balancer slotmem_create failed
2017:05:17-00:03:06 fw01 reverseproxy: [Wed May 17 00:03:06.207960 2017] [:emerg] [pid 21497:tid 4152121024] AH00020: Configuration Failed, exiting

The fix was this:
https://community.sophos.com/kb/en-us/121309

Everything is now green again, but I don't understand why - it's an old bug?!