I wasn't thinking about error but lags in the startup of FTL, especially while counting/compiling regex entries after a reload.
I'm not sure if you are aware of this topic: DNS resolution unexpectedly stops after upgrade to 5.0 - #38 by yubiuser
If you don' have any debug flag enabled, you would see a delay between those two lines
[2020-07-02 11:41:59.853 1202] Reloading DNS cache
[2020-07-02 11:43:26.199 1202] Compiled 0 whitelist and 0 blacklist regex filters in 5.9 msec
With advanced debug flags it appears that pihole hangs at counting domains in vw_gravity
Querying count of distinct domains in gravity database table vw_gravity
And using the special debug branch it turned out to be here
[2020-07-17 17:23:44.892 28893M] Reading regex from database...
[...]
[2020-07-17 17:24:37.655 28893M] Querying count of distinct domains in gravity database table vw_regex_blacklist: SELECT COUNT(DISTINCT domain) FROM vw_regex_blacklist
It seems to be a tough bug, so far only one person encountered it on master but on new/regex I could reproduce it.