Microsoft released Rollup 3 for Microsoft Forefront Threat Management Gateway (TMG) 2010 Service Pack 2.
For Exchange, this Rollup fixes unexpected authentication prompts when using OWA published using Forefront Threat Management Gateway (TMG) 2010 in conjunction with RSA authentication and Forms-based Authentication (2783345)
2700248 A server that is running Forefront Threat Management Gateway 2010 may stop accepting all new connections and may become unresponsive
2761736 All servers in a load-balanced web farm may become unavailable in Forefront Threat Management Gateway 2010
2761895 The Firewall service (WSPSRV.EXE) may crash when the firewall policy rules are reevaluated in Forefront Threat Management Gateway 2010
2780562 PPTP connections through Forefront Threat Management Gateway (TMG) 2010 may be unsuccessful when internal clients try to access a VPN server on the external network
2780594 A non-web-proxy client in a Forefront Threat Management Gateway (TMG) 2010 environment cannot open certain load-balanced websites when TMG HTTPS inspection is enabled
2783332 You cannot log on when FQDN is used and Authentication delegation is set to “Kerberos constrained delegation” in a Forefront Threat Management Gateway 2010 environment
2783339 A closed connection to a domain controller is never reestablished when Authentication delegation is set to “Kerberos constrained delegation” in a Forefront Threat Management Gateway 2010 environment
2783345 Unexpected authentication prompts while you use an OWA website that is published by using Forefront Threat Management Gateway (TMG) 2010 when RSA authentication and FBA are used
2785800 A “DRIVER_IRQL_NOT_LESS_OR_EQUAL (d1)” Stop Error may occur on a server that is running Forefront Threat Management Gateway (TMG) 2010
2790765 A “Host Not Found (11001)” error message occurs when an SSL site is accessed by using a downstream Forefront Threat Management Gateway 2010 server that has HTTPS Inspection enabled
For TMG, support will end on April 14th, 2015 and extended support will end on April 14th, 2020. You have some time to look into alternatives.
You can request Forefront TMG SP2 RU3 directly from support here.