after some package update and reboot (installation on ubuntu 18.04 server), i can see in my web admin panel - status check :
The IP address of this machine XX.XX.XX.XX is listed in the Spamhaus Block List (code [timeout]), which may prevent recipients from receiving your email. See http://www.spamhaus.org/query/ip/XX.XX.XX.XX.
Probably just a timeout when querying spamhaus. If I remember correctly the wait period in 0.50 is longer though perhaps you are on that version already.