Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 18.0.2): 18.0.4.2
Operating system and version (eg, Ubuntu 20.04): ubuntu server 18.04.4
Apache or nginx version (eg, Apache 2.4.25): NGinx 1.17.10
PHP version (eg, 7.1): 7.4.5
The issue you are facing: Trying to access array offset on value of type bool at /var/www/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php#1323
Is this the first time you’ve seen this error? (Y/N): yes since updating php7.3-fpm to php7.4-fpm
Steps to replicate it:
- use Nextcloud 18.x
- make changes to nginx.conf (using the socket of php7.4 instead of 7.3)
- change systemwide prefered php version from php7.3-fpm to php7.4-fpm by using following command: sudo update-alternatives --config php
- reboot nextcloud server
- check logfiles for errors
The output of your Nextcloud log in Admin > Logging:
Trying to access array offset on value of type bool at /var/www/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php#1323