Serial chart says that the com port is busy when its not about serialchart HOT 8 CLOSED

Same problem with Windows 7 64-bit

Same problem as well, Windows 7 64-bit
Getting error message:
"Could not open port COM15
Make sure the port is available and not used by other applications"

No other apps are using COM15 when attempted to run and confirmed communication 
is good on COM15 by using Arduino's serial monitor and a RS232 Data logger.

Tested with XP and works

Exactly identical error; using Windows 7 64-bit
error message:  "Could not open port COM15
Make sure the port is available and not used by other applications"

made sure that no other software instances were blocking the use of Serial Port.
Cross-checked with Free Serial Port Monitor 4.0

[deleted comment]

I receive the same error.. It seems that the higher com port numbers receive 
the message when the lower (below 10) do not.  The program seems to work on com 
ports like 3, 4, etc..

(Windows 7 64bit)
error message:  "Could not open port COM12 Make sure the port is available and 
not used by other applications"
Also made sure that no other software instances were blocking the use of Serial 
Port.

Noticed same behavior as listed by markc... (comment 5). Program works with 
lower numbered serial ports. In my case, did not work on com11 and worked when 
port # changed to com2.

Same problem wit win7 64 but changing the port to 2 as noted in comment 6 works.

This a problem that comes from the qt serial library. So I will not be fixing 
that but as a workaround I want to let you know that you're not stuck with 
high-numbered ports that Windows assigns , you can change them to something 
lower than 10, for example COM7 , COM5, here is how:

Right Click on My Computer, and click Manage
Then from Device Manager > Ports (COM & LPT)
Right click the port with high number and choose Properties.
Click on Port Settings Tab and then click Advanced...
On that dialog you can change COM Port Number to a number lower than 10. 
Sometimes it will say "In Use" but you can assign it over it , as long as that 
device is not connected (you don't see it under device manager).