The cause of this problem is that multiple SendKeys are being called in quick succession.
http://support.microsoft.com/kb/179987
The MATRIX add-in only has one SendKeys statement, so it is actually the interaction between this add-in and some other add-in with SendKeys statement(s) that is causing the problem.
The following modification removes the SendKeys statement from the MATRIX add-in and
might resolve this issue for you.
1. Open Excel and open the VBA editor (ALT-F11)
2. In the project explorer panel up the top left...
2. a. Expand VBAMatrix (matrix.xla)
2. b. Expand Modules folder under VBAMatrix
2. c. Double click on Initialize
3. Press CTRL-F and search for "SendKeys" (without the quotation marks!)
4. Replace the following line of code
Application.SendKeys "%^{F9}"
with
'Application.SendKeys "%^{F9}"
Application.CalculateFull