I have the same problem with RB751-U-2nHD running on 5.8
In my case web proxy is enabled but cache is set to 0. I use it only to block certain URLs.
I've noticed that router freezes from time to time. At the same time when it freezes flash usage goes up to 100%.
> /ip proxy export
set always-from-cache=no cache-administrator="" cache-hit-dscp=4 \
cache-on-disk=no enabled=yes max-cache-size=none max-client-connections=\
600 max-fresh-time=3d max-server-connections=600 parent-proxy=0.0.0.0 \
parent-proxy-port=0 port=8080 serialize-connections=no src-address=\
/ip proxy access
add action=deny disabled=no dst-host=... redirect-to=....
Previously I was running the same configuration on RB150 and had no issues.