The mbstring extension is recommended but optional in the environment checks, but the LTI activity fails if it isn't installed, with the following message:

Exception - Call to undefined function Html2Text\mb_internal_encoding()

More information about this error
Debug info:
Error code: generalexceptionmessage
Stack trace:

line 327 of \lib\html2text\Html2Text.php: Error thrown
line 277 of \lib\html2text\Html2Text.php: call to Html2Text\Html2Text->convert()
line 1915 of \lib\weblib.php: call to Html2Text\Html2Text->getText()
line 364 of \mod\lti\locallib.php: call to html_to_text()
line 172 of \mod\lti\locallib.php: call to lti_build_request()
line 262 of \mod\lti\locallib.php: call to lti_get_launch_data()
line 68 of \mod\lti\launch.php: call to lti_launch_tool()

It seems to me that either the mbstring extension should be required, or the Html2Text library should be customised to avoid failing when it is not installed.