This has been a problem for years.
De-ice fluid used - which dries in "dead airflow" area's, usually in the shrouds fwd of the ailerons and elevators.
What is left is a grey powder residue, which when rain is added swells up into a gel again, but this time without the anti-ice properties, so take off into icing conditions and the watery gel freezes and "glues" the control surface.
Control regained when a/c no longer in icing conditions.
The company I work for has frequent checks from Oct to May (inclusive) to check for the residue, if found it is brushed away and flushed until no gel formation is apparent.
When it re-hydrates it is quite an impressive sight, easy to see how it swells to fill these dead areas.
Pain in the backside xx feet up in a bucket at 2 in the morning on a wet winters night, but better that than presenting the pilot with a problem in flight.
Sounds like some other companys could do well to adopt the same checks.