I am facing the similar situation of ocassionally being unable to open FDTD file using python api in the ubuntu linux server. I run the python code in a seperate python IDE other than built-in python version. When my python code opens FDTD file at first time, it normally opens the file, however, when python code open FDTD files sequentially, Opening error usually occurs.


python code in cmd 

>>> from importlib.machinery import SourceFileLoader
>>> api_path = "/opt/lumerical/v231/api/python/lumapi.py"
>>> lumapi = SourceFileLoader("lumapi", api_path).load_module()
>>> fdtd = lumapi.FDTD(hide =True)
Traceback (most recent call last):
  File "", line 1, in
  File "/opt/lumerical/v231/api/python/lumapi.py", line 1394, in __init__
    super(FDTD, self).__init__('fdtd', filename, key, hide, serverArgs, **kwargs)
  File "/opt/lumerical/v231/api/python/lumapi.py", line 1112, in __init__
    self.handle = open(product, key, hide, serverArgs)
  File "/opt/lumerical/v231/api/python/lumapi.py", line 242, in open
    raise LumApiError(error)
lumapi.LumApiError: 'Exception [::appOpened]: Session not found'

Also, I have same issue when using lumapi in vs code.
I would appreciate your help.
Best regards,
Junhyung Park