PHP Fatal Error on Template

Hi there,
I have an issue with Mailpoet 2, with subscription forms in particular.

Both in the frontend and in the backend, the forms won’t load and will crash the page, reporting a PHP Fatal that I copy here below:

[Wed Sep 18 12:42:46.194215 2019] [php7:warn] [pid 17750] [client 94.38.239.143:56920] PHP Warning: preg_match(): Compilation failed: invalid range in character class at offset 11 in /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php on line 526,
[Wed Sep 18 12:42:46.194279 2019] [php7:warn] [pid 17750] [client 94.38.239.143:56920] PHP Warning: preg_match(): Compilation failed: invalid range in character class at offset 11 in /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php on line 526,
[Wed Sep 18 12:42:46.194291 2019] [php7:warn] [pid 17750] [client 94.38.239.143:56920] PHP Warning: preg_match(): Compilation failed: invalid range in character class at offset 11 in /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php on line 526,
[Wed Sep 18 12:42:46.194440 2019] [php7:error] [pid 17750] [client 94.38.239.143:56920] PHP Fatal error: Uncaught Exception: Template ‘templates/form/web/widgets/#type.html’ not found ! in /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php:723\nStack trace:\n#0 /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php(454): WYSIJA_help_render_engine->_loadTemplate(‘templates/form/…’)\n#1 /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php(169): WYSIJA_help_render_engine->_parse(‘\\n{include file=…’)\n#2 /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/form_engine.php(756): WYSIJA_help_render_engine->render(Array, ‘templates/form/…’)\n#3 /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/form_engine.php(638): WYSIJA_help_form_engine->render_web_body()\n#4 /var/www/html/public_html/wp-content/plugins/wysija-newsletters/views/front/widget_nl.php(160): WYSIJA_h in /var/www/html/public_html/wp-content/plugins/wysija-newsletters/helpers/render_engine.php on line 723,

I do not want to upgrade to version 3 as it does not migrate past newsletters (already tried).

Thanks! ??
Have a nice day,
Stefano