ProxyPass ttl and error messages on backend server

[Matthias Leopold <matthias@xxxxxxxxxxxxxxx>]

hi,

i'm running typo3 behind a reverse proxy. both frontend and backend 
servers are apache httpd on linux. now we moved the backend server from

apache 2.2
mod_php PHP 5.3
Typo3 4.5

to

apache 2.4
php-fpm PHP 5.6
Typo3 7.2

this brought unexpected woes with the reverse proxy, which had to be 
upgraded to apache 2.4 too. now i'm seeing lots of [ssl:info] error 
messages on the backend server, which i didn't have in the old setup. 
there's no visible problem with the frontend service, i just want know 
where these messages exactly stem from and whether they are ok.

messages are
(70007)The timeout specified has expired
(104)Connection reset by peer
(70014)End of file found

for reasons unknown to me i had to set a ttl on the ProxyPass directive 
on the frontend or else i would be having frequent 502 errors. this only 
worked with apache 2.4 (ttl below KeepAliveTimeout of backend server), 
this is why i switched to 2.4 on the frontend too.

additionally i had to switch from Content-Length header usage in typo3 
to Chunked transfer encoding or else i would frequently be having a 
damaged http header and source code output when accessing the frontend

frontend is https only and uses "SSLProxyEngine On"
frontend uses KeepAlive on

my question now is:
are these frequent error messages in the backend as mentioned above only 
the result of the ttl usage or are they a sign of misconfiguration? can 
somebody give me a hint why i suddenly had to use a ttl on the ProxyPass?

thanks a lot for reading so far, i hope someone can give me some 
reasonable feedback

matthias

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