could not open port 'COM3': WindowsError(5, 'Access is denied.')

I am using Sikuli to execute a small .exe app written from python that communicates with an Arduino board. The board emulates a true HID mouse via a serial port, and the .exe file tells the board to send a mouseclick.

For several weeks everything was working perfectly until recently when I started to intermittently get an error that the COM port could not be opened. This happens only when I execute the app through Sikuli. When I run the app from command line or double-click directly, it functions without error.

Any idea why this occurs?

This is the command window output:

Microsoft Windows [Version 10.0.17763.973]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\Users\D\Desktop\LPM>[u'COM3']
[u'COM3']
[u'COM3']
[u'COM3']
[u'COM3'[u'COM3']
][[
uu'COM3'[u'COM3']
Traceback (most recent call last):
  File "click.py", line 14, in <module>
  File "site-packages\serial\serialwin32.py", line 31, in __init__
  File "site-packages\serial\serialutil.py", line 240, in __init__
  File "site-packages\serial\serialwin32.py", line 62, in open
serial.serialutil.SerialException: could not open port 'COM3': WindowsError(5, 'Access is denied.')
[2636] Failed to execute script click
'COM3']]

Traceback (most recent call last):
[ File "click.py", line 14, in <module>
[u[u'COM3']
  File "site-packages\serial\serialwin32.py", line 31, in __init__
'COM3'[u File "site-packages\serial\serialutil.py", line 240, in __init__
]u['COM3'[ File "site-packages\serial\serialwin32.py", line 62, in open
userial.serialutil[u]
'COM3''COM3'.]'COM3'
]Traceback (most recent call last):
SerialException
]u
  File "click.py", line 14, in <module>
: File "site-packages\serial\serialwin32.py", line 31, in __init__

'COM3'Traceback (most recent call last):
Traceback (most recent call last):
c File "click.py", line 14, in <module>
] File "site-packages\serial\serialwin32.py", line 31, in __init__
Traceback (most recent call last):
  File "site-packages\serial\serialutil.py", line 240, in __init__
  File "click.py", line 14, in <module>
  File "click.py", line 14, in <module>

  File "site-packages\serial\serialutil.py", line 240, in __init__
ould not open port 'COM3': WindowsError(5, 'Access is denied.') File "site-packages\serial\serialwin32.py", line 62, in open
  File "site-packages\serial\serialwin32.py", line 31, in __init__
  File "site-packages\serial\serialwin32.py", line 31, in __init__
Traceback (most recent call last):
  File "site-packages\serial\serialwin32.py", line 62, in open

[10056] Failed to execute script click
serial.serialutilserial.serialutil File "site-packages\serial\serialutil.py", line 240, in __init__
  File "site-packages\serial\serialutil.py", line 240, in __init__
  File "click.py", line 14, in <module>
.. File "site-packages\serial\serialwin32.py", line 62, in open
  File "site-packages\serial\serialwin32.py", line 62, in open
  File "site-packages\serial\serialwin32.py", line 31, in __init__
SerialExceptionSerialExceptionserial.serialutilserial.serialutil File "site-packages\serial\serialutil.py", line 240, in __init__
: : .. File "site-packages\serial\serialwin32.py", line 62, in open
ccSerialExceptionSerialExceptionserial.serialutilould not open port 'COM3': WindowsError(5, 'Access is denied.')ould not open port 'COM3': WindowsError(5, 'Access is denied.'): : .
SerialExceptioncc
[8356] Failed to execute script click
ould not open port 'COM3': WindowsError(5, 'Access is denied.')ould not open port 'COM3': WindowsError(5, 'Access is denied.')[9896] Failed to execute script click

[15636] Failed t
[o: 3116] execute script click
Failed to execucte script click
ould not open port 'COM3': WindowsError(5, 'Access is denied.')
[11324] Failed to execute script click