I have an issue with syncing to AD with UPSS. The service starts and this account was used on this farm, in this domain prior to a rebuild. After rebuilding, I am able to add sync connection, populate the containers and select my users, however, when I select to start a sync (full or incremental) the job never starts...no errors, no profiles, the status continues to show IDLE.

Service is running, sync account is in local admin of upss server....

Sync Account does not need to be Local Admin. Instead, the Farm Admin account needs to be Local Admin (and also the user running the UPSS).

So... just found bigger issue... 

On my farm, when users login, they get 500 error... if you login as farm account, the page loads... must be related...

seeing the following on primary app server

A failure was reported when trying to invoke a service application: EndpointFailure
Process Name: OWSTIMER
Process ID: 9912
AppDomain Name: DefaultDomain
AppDomain ID: 1
Service Application Uri: urn:schemas-microsoft-com:sharepoint:service:01496931975646de8300fa3ae55f48f9#authority=urn:uuid:97fb865c161d4842bb360d7b6dbc5889&authority=https://rchspapt1:32844/Topology/topology.svc
Active Endpoints: 1
Failed Endpoints:1

Check the Application Event Log and ULS log. May also want to examine the IIS log for that site.

is their any end point failure for userprofile?

There is not.... this is what I am seeing:

Failure trying to synch web application 6c2d88f6-55c0-4d69-88a1-3f0270018a33, ContentDB 38587513-de71-465c-b62e-4910026e4e18  Exception message was Microsoft.Office.Server.UserProfiles.UserProfileApplicationNotAvailableException: UserProfileApplicationNotAvailableException_Logging :: UserProfileApplicationProxy.ApplicationProperties ProfilePropertyCache does not have 625c9cec-6e26-4fee-b09e-b1fae12ea2e7
   at Microsoft.Office.Server.Administration.UserProfileApplicationProxy.get_ApplicationProperties()
   at Microsoft.Office.Server.Administration.UserProfileApplicationProxy.get_PerfmonInstanceHandle()
   at Microsoft.Office.Server.UserProfiles.ContentDBSynchronizer..ctor(ELogType logType, SPContentDatabase cdb, SPJobState jobState, SyncType syncType, WSSSyncUsageEntry usage, String[] excludedAccountsRegexStrings)
   at Microsoft.Office.Server.UserProfiles.WSSProfileSyncJob.Execute()

Found this:

Failure trying to synch web application 6c2d88f6-55c0-4d69-88a1-3f0270018a33, ContentDB 38587513-de71-465c-b62e-4910026e4e18  Exception message was Microsoft.Office.Server.UserProfiles.UserProfileApplicationNotAvailableException: UserProfileApplicationNotAvailableException_Logging :: UserProfileApplicationProxy.ApplicationProperties ProfilePropertyCache does not have 625c9cec-6e26-4fee-b09e-b1fae12ea2e7
   at Microsoft.Office.Server.Administration.UserProfileApplicationProxy.get_ApplicationProperties()
   at Microsoft.Office.Server.Administration.UserProfileApplicationProxy.get_PerfmonInstanceHandle()
   at Microsoft.Office.Server.UserProfiles.ContentDBSynchronizer..ctor(ELogType logType, SPContentDatabase cdb, SPJobState jobState, SyncType syncType, WSSSyncUsageEntry usage, String[] excludedAccountsRegexStrings)
   at Microsoft.Office.Server.UserProfiles.WSSProfileSyncJob.Execute()

Try this...

stsadm -o sync -deleteolddatabases 0

That removed two db's..however, UPSS still will not start the import...

Trevor,

I ran the stsadm and it did remove two old db's... however, I am still receiving the error.

Also, fwiw, I tried running the stsadm cmd from my 2nd app server and it failed...referred me to diagnostic logs... which I have yet to look at, as I ended up successfully running from my primary app server.

A failure occurred during the processing of this command. Check diagnostic logs
for more information.

More info, the two content db's in this farm were both restored from a lower environment (integration >  qa)... wondering if there are permission issues on those dbs....

Please check the ULS logs. This command shouldn't fail unless there is a database schema issue or UPA Proxy issue.

try to recycle the STS app pool on the servers where you are seeing the STS token issue...i think, reset on all server.