MS support requested to modify the time limit of a cross-forest AutoDiscover operation via the RecipientResolutionTimeoutInSeconds property (KB928812). Just adding the following line into Web.config was needed:
It was caused by an invalid form of double quotation marks (despite copy/paste through notepad).
Bear in mind to update config files carefully and double check related syntax. Use only supported double quotation marks " and omit ” ‟ “ ″ .