# If we receive a forwarded http request from a proxy...
RewriteCond %{HTTP:X-Forwarded-Proto} =http [OR]

# ...or just a plain old http request directly from the client
RewriteCond %{HTTP:X-Forwarded-Proto} =""
RewriteCond %{HTTPS} !=on

# Redirect to https version
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Link to original solution:
[url=https://stackoverflow.com/questions/36748110/htaccess-too-many-redirects-when-trying-to-force-https]Stackoverflow.com[/url]

Soluzione completa per attivare https su phpbb 3.2.x

1) attivare SSL / HTTPS presso il vostro provider / ISP
2) Pannello di controllo Admin--> GENERALE-->cookie-->cookie sicuri ABILITATI (impostare)

Attendere.

Nel frattempo: controllare che ogni link interno del vostro forum phpbb (banner ecc..) sia https:// o non sia assoluto, e che ogni link ESTERNO lo sia altrettanto

Di seguito la regola .htaccess che regolerà la redirezione verso le pagine sicure:

[code]# If we receive a forwarded http request from a proxy...
RewriteCond %{HTTP:X-Forwarded-Proto} =http [OR]

# ...or just a plain old http request directly from the client
RewriteCond %{HTTP:X-Forwarded-Proto} =""
RewriteCond %{HTTPS} !=on

# Redirect to https version
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301][/code]

Come ho detto, si ringrazia l'autore del post su Stack Overflow: [url=https://stackoverflow.com/users/1551102/harshal-lonare]Harshal Lonare[/url]