error when trying to run dash app in debug mode

Question

While trying to run the Dash app I'm working in debug mode, the following unexpected error is thrown:

 * Serving Flask app 'app'
 * Debug mode: on
Traceback (most recent call last):
  File "/..../anaconda3/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/..../anaconda3/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/...../my-app/app.py", line 1, in <module>
    import dash
ModuleNotFoundError: No module named 'dash'

Application works fine in run-mode. Debug-mode used to work as well. Can't figure out what might have changed. My app is a multi-page app with the following basic set-up in app.py (main page)

import dash
from dash import html
import dash_bootstrap_components as dbc

app = dash.Dash(__name__, use_pages=True, suppress_callback_exceptions=True)

...


if __name__ == "__main__":
    app.run(debug=True)

Hope my question is clear enough. Many thanks in advance.