Metatrader tends to copy DLL files located in the main program folder into the associated data folder whenever a new MT4 build is released.
If the plug-in you are using signals "Error - Incorrect DLL Version (20141008.1)" or similar, you should do the following:
Go into your mt4's data folder* and into MQL4/Libraries sub folder.
You'll need to delete the old duplicate DLL files. Depending on the plug-in you are using this will be one of the following (or both):
*) Data folder is accessed from MT4's File menu -> "Open Data Folder":