If you have Nextcloud running and discovered that Rainloop ist way better then the regular mail plugin for Nextcloud then you are on the right track.
However if the above warning keeps looking at you and you can not figure out why it is still looking at you, well let me try to explain.
This could either mean that your rainloop folder is in fact reachable from the outside and this is nothing you want to have happen. Funny enough it could mean you did secure it properly, but you are running into the fact that the warning being given here is based on a „check“ not being 100% true. Nextcloud is returning a 302 http code (found). The „check“ is thingking that this means that the „VERSION“ file can be read. However nextcloud is doing a rewrite and sending you elsewhere and that „elsewhere“ has been „found“. So the „check“ is telling you, the location is accessible one way or the other.
To understand a bit more about it, read the folling github Ticket: https://github.com/pierre-alain-b/rainloop-nextcloud/issues/62