Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger RT1.UserTrigger caused an unexpected exception, contact your administrator: RT1.UserTrigger: execution of AfterUpdate caused by: System.ExternalObjectException: External Object Error: (RT1)

When I try to make a change to a user, I recieve the above alert.  Technical support reccomeded I post here for help.  

The Winter 15 API has a change to the SOQL SELECT statement (https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_select.htm) that allows multiple FROM objects, but no description is given for the change.

FROM objectType[,objectType...]

The error message received indicates that it's to be used with related objects:

ERROR at Row:1:Column:21
A driving SObject type has already been set, all other entity types in the FROM clause must be relationships to the initial object.  The driving object is Case.

Can we get a description of this new syntax, or correct the docs if this is something hinted before release?  The only matching forum post was from a new dev trying to do the usual but misled by the doc bnf.

The Winter 15 API has a change to the SOQL SELECT statement (https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_select.htm) that allows multiple FROM objects, but no description is given for the change.

FROM objectType[,objectType...]

The error message received indicates that it's to be used with related objects:

ERROR at Row:1:Column:21
A driving SObject type has already been set, all other entity types in the FROM clause must be relationships to the initial object.  The driving object is Case.

Can we get a description of this new syntax, or correct the docs if this is something hinted before release?  The only matching forum post was from a new dev trying to do the usual but misled by the doc bnf.