New property from server.properties:

# The backend either delivers a thumbnail when it's available from cache or fails fast while initiating the thumbnail creation/caching in
# the background. The advantage is that clients aren't blocked and can simply retry later when the thumbnail can be delivered from cache.
#
# If there is no cache configured for the user/context:
# - we either have to generate the thumbnail on the fly which has the potential to block the whole client by occupying the max number of
# allowed connections to the domain
# - or simply fail fast which leaves the client responsive but without thumbnails
#
# Default value is false to keep the client responsive.
com.openexchange.preview.thumbnail.blockingWorker= false