HI, I had the same issue installing any app on Nextcloud, since I got this error:
❯ sudo docker exec -it nextcloud occ app:install notes
Error: cURL error 28: SSL connection timeout (see https://curl.haxx.se/libcurl/c/libcurl-errors.html)
once I’ve set the MTU on wireguard server to 1.500 I’ve fixed the issue. Thanks for your sharing!