Custom template don't work anymore

I have a package that uses Panel to create interactive plots. My last build of the package was done at 21 September 2023. Everything worked fine. I can’t tell the version of panel used on that build because Github logs expired. Based on my local environment, I guess it was 1.2.2.

This is the structure of the submodule that implements interactivity with panel. Note that I’m using a custom template based on bootstrap.

- interactive
  - bootstrap_spb: folder containing the template files
    - __init__.py: this contains my custom template class, `class SymPyBootstrapTemplate(BootstrapTemplate):`
    - bootstrap.css
    - bootstrap.html
  - __init__.py
  - panel.py: file containing the logic to create the interactive application, using the template contained into bootstrap_spb.

Now, I have updated my package and I need to release a new version. The updates didn’t changed the interactive submodule. However, with the latest version of panel my tests are failing with the following error:

ValueError: '/home/davide/Documents/Development/sympy_plot_backends/spb/interactive/bootstrap_spb/bootstrap.html' is not in the subpath of '/home/davide/Documents/Development/envs/plot/lib/python3.10/site-packages/panel/template' OR one path is relative and the other is absolute.

I went trough the documentation, but I didn’t find anything that could help me. What do I have to do to load my custom template with the latest panel version?

This is how I loaded the template (code from my file panel.py):

from spb.interactive.bootstrap_spb import SymPyBootstrapTemplate

# kwargs contains customization options
template = SymPyBootstrapTemplate(**kwargs)

# add the plot
template.main.append(self.pane)
# add the controls
template.sidebar.append(self.layout_controls)