Can view but not update calendar

Steps to reproduce

  1. This install of owncloud has been working for years. This problem suddenly started.
  2. Connect with web interface on desktop or DAV client from mobile
  3. View calendar
  4. Attempt to add a calendar entry

Expected behaviour

The existing calendar entries should be visible.
The new calendar entry should be added.

Actual behaviour

Existing calendar entries are visible
When I attempt to add a new item:
On desktop: Entry does not appear in calendar
On mobile: Fails and reports a Internal Server Error

Note, on desktop, I can create new calendars, but they also can’t be updated.
In the log I see:
“message”:“Could not get node for path: NAME OF NEW CALENDAR” : Node with name ‘NAME OF NEW CALENDAR’ could not be found"}

There are similar "Calendar object not found messages earlier in the log.

Server configuration

Operating system:
Ubuntu 18.04.4 LTS

Web server:
Apache 2.4.29

Database:
SQLite 3

PHP version:
7.2

ownCloud version: (see ownCloud admin page)
10.6.0.5 (problems originally seen on 10.4.0)

Updated from an older ownCloud or fresh install:
Problem seen on 10.4.0. Upgraded to 10.6.0 to see if that fixed it.

Where did you install ownCloud from:

Signing status (ownCloud 9.0 and above):

Login as admin user into your ownCloud and access 
http://example.com/index.php/settings/integrity/failed 
paste the results into https://gist.github.com/ and puth the link here.

“No errors have been found.”

The content of config/config.php:

Log in to the web-UI with an administrator account and click on
'admin' -> 'Generate Config Report' -> 'Download ownCloud config report'
This report includes the config.php settings, the list of activated apps
and other details in a well sanitized form.

or 

If you have access to your command line run e.g.:
sudo -u www-data php occ config:list system
from within your ownCloud installation folder

*ATTENTION:* Do not post your config.php file in public as is. Please use one of the above
methods whenever possible. Both, the generated reports from the web-ui and from occ config:list
consistently remove sensitive data. You still may want to review the report before sending.
If done manually then it is critical for your own privacy to dilligently
remove *all* host names, passwords, usernames, salts and other credentials before posting.
You should assume that attackers find such information and will use them against your systems.

List of activated apps:
calendar
contacts

If you have access to your command line run e.g.:
sudo -u www-data php occ app:list
from within your ownCloud installation folder.

Are you using external storage, if yes which one: local/smb/sftp/…

Are you using encryption: yes/no
No

Are you using an external user-backend, if yes which one: LDAP/ActiveDirectory/Webdav/…

Client configuration

Browser:

Operating system:

Logs

Web server error log

Insert your webserver log here

ownCloud log (data/owncloud.log)

Insert your ownCloud log here

Browser log

Insert your browser log here, this could for example include:

a) The javascript console log
b) The network log 
c) ...

Do the log messages indicate some kind of database corruption? That doesn’t entirely make sense since the existing entries are all visible. I also have found I can read but not write contact info. So that it is two separate apps with very similar issues.

Just for completeness sake, I’m updating this.

A couple of weeks after posting the question, things started magically working and have continued to do so since then.