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