[UAG] HTTP 500.19 error

On a Forefront UAG array deployment for some reason when for an array member I disjoin and rejoin it to the array I got the following error after the login page of the portal.

The error still remains even when I rebuild the IIS websites through the Forefront UAG activation process.

After some research I notice something weird on the IIS Application Pools node. For obscure reason the Pools used by Forefront UAG switched to .Net 4.

When I switched back to .Net Framework 2.0 the following Application Pools all work fine again:

• Classic .NET AppPool
• DefaultAppPool
• DomainHRAPool
• InternalSiteMobile
• NonDomainHRAPool
• Portal