in "Debug subsystems" you need to enter a comma separated list of which subsystems to debug:
# /home/builder/tvheadend_official/tvheadend/build.linux/tvheadend --subsystems
Subsystems:
START START
STOP STOP
CRASH CRASH
main Main
tprof Time profiling
qprof Queue profiling
CPU CPU
thread Thread
tvhpoll Poll multiplexer
time Time
spawn Spawn
fsmonitor Filesystem monitor
lock Locking
uuid UUID
idnode Node subsystem
url URL
tcp TCP Protocol
rtsp RTSP Protocol
upnp UPnP Protocol
settings Settings
config Configuration
access Access (ACL)
cron Cron
dbus DBUS
avahi Avahi
bonjour Bonjour
api API
http HTTP Server
httpc HTTP Client
htsp HTSP Server
htsp-sub HTSP Subscription
htsp-req HTSP Request
htsp-ans HTSP Answer
imagecache Image Cache
tbl DVB SI Tables
tbl-base Base DVB SI Tables (PAT,CAT,PMT,SDT etc.)
tbl-csa DVB CSA (descrambling) Tables
tbl-eit DVB EPG Tables
tbl-time DVB Time Tables
tbl-atsc ATSC SI Tables
tbl-pass Passthru Muxer SI Tables
tbl-satip SAT>IP Server SI Tables
fastscan Fastscan DVB
pcr PCR Clocks
parser MPEG-TS Parser
TS Transport Stream
globalheaders Global Headers
tsfix Time Stamp Fix
hevc HEVC - H.265
muxer Muxer
pass Pass-thru muxer
audioes Audioes muxer
mkv Matroska muxer
service Service
channel Channel
subscription Subscription
service-mapper Service Mapper
bouquet Bouquet
esfilter Elementary Stream Filter
profile Streaming Profile
descrambler Descrambler
descrambler-emm Descrambler EMM
caclient CA (descrambling) Client
csa CSA (descrambling)
capmt CAPMT CA Client
cwc CWC CA Client
cccam CWC CCCam Client
dvbcam DVB CAM Client
dvr Digital Video Recorder
dvr-inotify DVR Inotify
epg Electronic Program Guide
epgdb EPG Database
epggrab EPG Grabber
charset Charset
dvb DVB
mpegts MPEG-TS
muxsched Mux Scheduler
libav libav / ffmpeg
transcode Transcode
iptv IPTV
iptv-pcr IPTV PCR
iptv-sub IPTV Subcription
linuxdvb LinuxDVB Input
diseqc DiseqC
en50221 CI Module
en50494 Unicable (EN50494)
satip SAT>IP Client
satips SAT>IP Server
tvhdhomerun TVHDHomeRun Client
psip ATSC PSIP EPG
opentv OpenTV EPG
pyepg PyEPG Import
xmltv XMLTV EPG Import
webui Web User Interface
timeshift Timeshift
scanfile Scanfile
tsfile MPEG-TS File
tsdebug MPEG-TS Input Debug
codec Codec
vaapi VA-API
ddci DD-CI
Or you can use "all" for everything.
For more in depth debugging, you can enable "Debug trace (low-level):" and enter the subsystems as above.
You can also use negative values (ie, -epg) to remove things after an "all".
all,-epg
after applying it will return with a list of all subsystems except epg.