Here is the report:

--- BEGIN DEBUG INFO ---

EXCEPTION:
java.lang.RuntimeException: Unable to resume activity {at.bitfire.davdroid/at.bitfire.davdroid.ui.AccountSettingsActivity}: at.bitfire.davdroid.InvalidAccountException: Invalid account: Account {name=USERNAME, type=bitfire.at.davdroid}
	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3822)
	at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3854)
	at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51)
	at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loop(Looper.java:193)
	at android.app.ActivityThread.main(ActivityThread.java:6718)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: at.bitfire.davdroid.InvalidAccountException: Invalid account: Account {name=USERNAME, type=bitfire.at.davdroid}
	at at.bitfire.davdroid.settings.AccountSettings.<init>(AccountSettings.kt:109)
	at at.bitfire.davdroid.ui.AccountSettingsActivity$AccountSettingsFragment.reload(AccountSettingsActivity.kt:119)
	at at.bitfire.davdroid.ui.AccountSettingsActivity$AccountSettingsFragment.onResume(AccountSettingsActivity.kt:98)
	at androidx.fragment.app.Fragment.performResume(Fragment.java:2649)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:922)
	at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
	at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
	at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2659)
	at androidx.fragment.app.FragmentManagerImpl.dispatchResume(FragmentManagerImpl.java:2625)
	at androidx.fragment.app.FragmentController.dispatchResume(FragmentController.java:268)
	at androidx.fragment.app.FragmentActivity.onResumeFragments(FragmentActivity.java:479)
	at androidx.fragment.app.FragmentActivity.onPostResume(FragmentActivity.java:468)
	at androidx.appcompat.app.AppCompatActivity.onPostResume(AppCompatActivity.java:195)
	at android.app.Activity.performResume(Activity.java:7325)
	at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3814)
	... 11 more

SOFTWARE INFORMATION
* at.bitfire.davdroid 2.6-ose (308) from org.fdroid.fdroid.privileged
* org.dmfs.tasks 1.2.3 (77600) from org.fdroid.fdroid.privileged
* com.android.providers.contacts 9 (28)
* com.android.providers.calendar 9 (28)
* com.android.contacts 1.7.31 (10731)
* com.android.calendar 9 (28)
* ws.xsoh.etar 1.0.18 (20) from org.fdroid.fdroid.privileged

CONNECTIVITY (at the moment)
- [ Transports: WIFI Capabilities: NOT_METERED&INTERNET&NOT_RESTRICTED&TRUSTED&NOT_VPN&VALIDATED&NOT_ROAMING&FOREGROUND&NOT_CONGESTED&NOT_SUSPENDED Unwanted:  LinkUpBandwidth>=1048576Kbps LinkDnBandwidth>=1048576Kbps SignalStrength: -51]

CONFIGURATION
Power saving disabled: yes
android.permission.READ_CONTACTS: granted
android.permission.WRITE_CONTACTS: granted
android.permission.READ_CALENDAR: granted
android.permission.WRITE_CALENDAR: granted
org.dmfs.permission.READ_TASKS: granted
org.dmfs.permission.WRITE_TASKS: granted
android.permission.ACCESS_COARSE_LOCATION: denied
System-wide synchronization: automatically
Account: USERNAME Nextcloud
  Address book sync. interval: 240 min
  Calendar     sync. interval: 240 min
  OpenTasks    sync. interval: 240 min
  WiFi only: false
  [CardDAV] Contact group method: GROUP_VCARDS
  [CalDAV] Time range (past days): 90
           Manage calendar colors: true
           Use event colors: false

SQLITE DUMP
android_metadata
	|  locale |
	|  de_DE |
----------
room_master_table
	|  id | identity_hash |
	|  42 | a7554d1eb[…]485d63c |
----------
service
	|  id | accountName | type | principal |
	|  5 | USERNAME Nextcloud | carddav | https://server.example.com:1234/nextcloud/remote.php/dav/principals/users/USERNAME/ |
	|  6 | USERNAME Nextcloud | caldav | https://server.example.com:1234/nextcloud/remote.php/dav/principals/users/USERNAME/ |
----------
sqlite_sequence
	|  name | seq |
	|  service | 6 |
	|  homeset | 6 |
	|  collection | 12 |
----------
homeset
	|  id | serviceId | url | privBind | displayName |
	|  5 | 6 | https://server.example.com:1234/nextcloud/remote.php/dav/calendars/USERNAME/ | 1 | <null> |
	|  6 | 5 | https://server.example.com:1234/nextcloud/remote.php/dav/addressbooks/users/USERNAME/ | 1 | <null> |
----------
collection
	|  id | serviceId | type | url | privWriteContent | privUnbind | forceReadOnly | displayName | description | color | timezone | supportsVEVENT | supportsVTODO | supportsVJOURNAL | source | sync |
	|  9 | 6 | CALENDAR | https://server.example.com:1234/nextcloud/remote.php/dav/calendars/USERNAME/SOMECAL1/ | 1 | 1 | 0 | NAMEOFCALENDAR1ö | <null> | <null> | <null> | 1 | 1 | 0 | <null> | 0 |
	|  10 | 6 | CALENDAR | https://server.example.com:1234/nextcloud/remote.php/dav/calendars/USERNAME/SOMECAL2/ | 0 | 0 | 0 | NAME OF CALENDAR 2 | <null> | -54 | <null> | 1 | 0 | 0 | <null> | 0 |
	|  11 | 6 | WEBCAL | https://server.example.com:1234/nextcloud/remote.php/dav/calendars/USERNAME/httpswww[…]lSOMECAL3ics/ | 1 | 1 | 0 | *** | <null> | -1605516 | <null> | 1 | <null> | <null> | https://www.[…]/static/ical/SOMECAL3.ics | 0 |
	|  12 | 5 | ADDRESS_BOOK | https://server.example.com:1234/nextcloud/remote.php/dav/addressbooks/users/USERNAME/contacts/ | 1 | 1 | 0 | Kontakte | <null> | <null> | <null> | <null> | <null> | <null> | <null> | 1 |
----------

SYSTEM INFORMATION
Android version: 9 (lineage_kiwi-userdebug 9 PQ3A.190801.002 eng.root.20191106.083120 dev-keys)
Device: HUAWEI KIW-L21 (kiwi)

--- END DEBUG INFO ---

LineageOS 16

STR:

  1. Add Nextcloud account
  2. Try to enable “Use event colors”
    -> it crashes

Note this happens in conjunction with this other bug, so maybe they are related:
https://forums.bitfire.at/topic/2170/duplicate-calenders-and-no-calender-syncronisation-with-nextcloud