After upgrading from v2 to v3, Jackett indexer not working

System Information/Logs

Sonarr Version:
v3.0.3.690
Operating System:
Ubuntu 18.04.4 LTS \n \l
.net Framework (Windows) or mono (macOS/Linux) Version:

mono --version
Mono JIT compiler version 6.8.0.105 (tarball Tue Feb  4 21:20:20 UTC 2020)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
	TLS:           __thread
	SIGSEGV:       altstack
	Notifications: epoll
	Architecture:  amd64
	Disabled:      none
	Misc:          softdebug
	Interpreter:   yes
	LLVM:          yes(610)
	Suspend:       hybrid
	GC:            sgen (concurrent by default)

Link to Log Files (debug or trace):

20-2-7 19:50:19.5|Debug|Api|[GET] /api/system/status?apikey=(removed) 200.OK (0 ms)
20-2-7 19:50:19.5|Debug|Api|[GET] /api/system/status?apikey=(removed) 200.OK (0 ms)
20-2-7 19:50:19.5|Debug|Api|[GET] /api/system/status?apikey=(removed) 200.OK (0 ms)
20-2-7 19:50:25.8|Debug|NewznabCapabilitiesProvider|Failed to parse newznab api capabilities for http://127.0.0.1:9117/jackett/api/v2.0/indexers/scenehd/results/torznab/

[v3.0.3.690] System.Xml.XmlException: Unexpected XML
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider.ParseCapabilities (NzbDrone.Common.Http.HttpResponse response) [0x00364] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:162
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider.FetchCapabilities (NzbDrone.Core.Indexers.Newznab.NewznabSettings indexerSettings) [0x0009d] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:67

20-2-7 19:50:25.8|Warn|Torznab|Unable to connect to indexer

[v3.0.3.690] System.Xml.XmlException: Unexpected XML
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider.ParseCapabilities (NzbDrone.Common.Http.HttpResponse response) [0x00364] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:162
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider.FetchCapabilities (NzbDrone.Core.Indexers.Newznab.NewznabSettings indexerSettings) [0x000d8] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:74
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider+<>c__DisplayClass4_0.<GetCapabilities>b__0 () [0x00000] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:35
  at NzbDrone.Common.Cache.Cached`1[T].Get (System.String key, System.Func`1[TResult] function, System.Nullable`1[T] lifeTime) [0x00069] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Common\Cache\Cached.cs:81
  at NzbDrone.Core.Indexers.Newznab.NewznabCapabilitiesProvider.GetCapabilities (NzbDrone.Core.Indexers.Newznab.NewznabSettings indexerSettings) [0x00020] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Newznab\NewznabCapabilitiesProvider.cs:35
  at NzbDrone.Core.Indexers.Torznab.Torznab.get_PageSize () [0x00000] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Torznab\Torznab.cs:23
  at NzbDrone.Core.Indexers.Torznab.Torznab.GetRequestGenerator () [0x00000] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\Torznab\Torznab.cs:27
  at NzbDrone.Core.Indexers.HttpIndexerBase`1[TSettings].TestConnection () [0x00007] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Indexers\HttpIndexerBase.cs:334

20-2-7 19:50:25.8|Warn|SonarrErrorPipeline|Invalid request Validation failed:
 -- : Unable to connect to indexer, check the log for more details
20-2-7 19:50:25.8|Debug|Api|[POST] /api/v3/indexer/test: 400.BadRequest (21 ms)

Browser (for UI bugs):

Seems like scenehd is not able to be used with jackett, and the information that is being presented is not being liked for some reason.

If I re-add the indexer with the same settings, and test - the indexer works fine.

Which leads me to believe that there is an issue with upgrading from v2 to v3 with jackett as an indexer.

Are you using a really old (or new) version of Jackett?

Lots of people are running v3 with Jackett.

Trace logs may show a snippet of the XML when it fails, but not 100% sure that it does.

Ive used Jackett since day one with Sonarr on v3. Have a half dozen indexers in there (some for automatic searching and some for manual).

Sonarr Version 3.0.3.698
Radarr Version 0.2.0.1450
Jackett Version 0.12.1804.0

That is my current setup and hums along

jackett (or maybe it sonarr) has had that issue for a very long time (well over a year)

i lose indexers all the time and running the test in sonarr typically brings them back, or running the test in jacket and then sonarr, but sometimes i have to restart jackett to get them to work again