LDAP login works but not SSO

[rthouvenin]

Describe the bug

When I am disconnected:

• I can login to Piwigo with my Yunohost LDAP credentials, this works fine.
• But if I connect to the Yunohost portal and click on the Piwigo tile, I am not connected to Piwigo: SSO does not work

Context

• Hardware: Server bought online
• YunoHost version: 4.2.8
• ssowat: 4.2.4
• Piwigo: 12.1.0
• Piwigo is on a sub-domain of the portal domain (I imagine this matters for the SSO cookie)

Let me know if anything else would be useful!

Steps to reproduce

See bug description.

Expected behavior

Logging into the Yunohost portal should log me in on all apps that support SSO. I'm pretty confident this used to work for Piwigo.