This is based on my experience today. Feel free to add errors and what you did to fix them.

Error #1

(20014)Error string not specified yet: Error retrieving pid file /data/wre/var/logs/modproxy.pid

Possible Solution #1

delete existing /data/wre/var/logs/modproxy.pid and restart modproxy

Error #2

[warn] Init: Session Cache is not configured [hint: SSLSessionCache]

Possible Solution #2

http://www.webgui.org/install/upgrade_help/sslsessioncache-help

http://www.webgui.org/install/upgrade_help/problems-with-wremonitor