This issue was addressed within the release candidate cycle.
PHP issue
But, your issue is a different one. Have you considered searching the forum and internet for that php error?
Here is a random Nextcloud setup guide from the internet
Warning 2: Small PHP memory limit
The PHP memory limit is below the recommended value of 512MB.
To fix this we need to edit the ‘php.ini’ file to increase the maximum amount of memory that the PHP process can consume. While we’re there, we might as well increase the maximum upload limit too.
sudo nano /etc/php/7.2/fpm/php.ini
Now look for the following values within the php.ini file and change them as needed. You can increase the PHP memory limit above 512MB if you wish.
On my production server, I have the PHP memory limit set to 1GB, as my server has 4GB RAM.
# Default is 128M
memory_limit = 512M
# Default is 2M
upload_max_filesize = 100M
# Restart PHP service again
sudo systemctl reload php7.2-fpm.service
Restart PHP again to apply the new settings and head back to the admin overview screen. Hit refresh and hey presto! Two of the warnings are already gone. Life is getting better all the time!