This error, 4106 ERR_UNKNOWN_SYMBOL, occurred when the chart, where an EA compiled on MetaEditor 4.00 Build 409 (25 Oct 2011) running on the latest MetaTrader 4.00 Build 610 (14 Feb 2014), had its timeframe manually changed from one to another, e.g. H1 to H4. Reverting back to the launch timeframe did not resolve the issue.
The error was resolved when the same .mq4 was compiled in the latest MetaEditor 5.00 Build 887 (13 Feb 2014)
No dll’s were involved.
Just throwing it out there for anyone that might be facing a similar issue.
NB: No doubt someone would be thinking why am I using such an old MetaEditor build and I would have to reply that if it ain’t broke, don’t fix it. It’s broke now though. No aspersions on the new MetaEditor, I think it’s a much better IDE and timely in dragging me into OOP.