Not able to save a calendar event

We’ve installed NextCloud into a server in which an OwnCloud has run before. So, we expected we could get NextCloud running too. We needed to upgrade the php to 7.2 during the installation but after that all went smoothly, no install errors or complains.

We mainly need the installation for Calendar sync. It was unclear if the Web app Calendar is needed if just using the NextCloud as a syncing server instance but apparently it is needed in order to have the Calendar account set up. So, we added Calendar app but we have issues with that one. We can’t save any event there, neither iOS or Mac is finding our calendar if we try to connect into it.

There are no server error logs regarding this issue. The only errors we can see are in browser Javascript Console:

Failed to load resource: the server responded with a status of 403 (Forbidden)

Unhandled Promise Rejection: [object Object]

Attempting to call a FullCalendar method on an element with no calendar.
warn — vendor.js:23910

The escapeHTML library is deprecated! It will be removed in nextcloud 19.

We have been reading the support forum and thus based on it, we also turned off Mod_Security but that doesn’t make any difference. We still get the exactly same error messages.

How could we debug this situation? It’s to be noted that we also installed Desk and those cards we can save without any errors even if it is similar actions. Although it looks that description for the card is not staying saved.

ADDED: Just tested also adding files, and it comes the same error about “Failed to load resource: the server responded with a status of 403 (Forbidden)

Anyway, the calendar sync is the thing we need.