today I want to share one workaround, which I sometime use as temporary measure for migration projects. Imagine, that you've added reference to your bin folder of one of your old customizations. Then you build it, and you've got this error message:
Server Error in '/xxx' Application.
Could not load file or assembly 'xxxx' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
This error happens because you have in your customization C# part references to some older dll's which are not compatible with current version of Acumatica. So ideal solution would be to re-add to your current class library proper dll.
Another TEMPORARY measure would be add something like this to your Post-build event command line:
command similar to this:
xcopy /Y /I "d:\SourceCode\build20r1core\KNMCCore\obj\Debug\KNMCCore.*" "c:\Program Files\Acumatica ERP\Matrix\Bin\"
xcopy says to visual studio please make copy
/Y - don't ask for confirmation
/I - copy multiple items
and then goes two string parameters - source with it's mask, and destination.