We had to manually exclude the oab.xml from the Web Acceleration profile "*_caching_profile" (*=name of iApp instance) created by the iApp and this resolved our issue. Here's what our profile looks like from bigip.conf:
ltm profile web-acceleration /Common/MSExchange.app/MSExchange_caching_profile {
app-service /Common/MSExchange.app/MSExchange
applications none
cache-uri-exclude { oab.xml }
cache-uri-include { .* }
cache-uri-include-override none
cache-uri-pinned none
defaults-from /Common/optimized-caching
}
Opened case with F5 and support confirmed this fix.
Nimbostratus