It seems to be related to the Dashboard tour?! I googled lots and found no other mention of the specific issue so wasn't really sure what to do...
Perhaps it was the wrong approach, but I posted a bug report. I didn't get any feedback so ended up abandoning my server.
Now a month later, I just tried again today, and hit exactly the same issue?! I find it bizarre that I seem to be able to reliably recreate this issue but no one else appears to have reported it?!
It makes me wonder whether I have a missing dependency (e.g. php module), incorrect permissions or perhaps some incorrect config?
This time I tried setting really permissive permissions and that appeared to make no difference. I also enabled debugging, and as noted on my bug report, I got a stacktrace which might be of more value than the error message?!:
Error code: generalexceptionmessage
* line 117 of /admin/tool/usertours/classes/target.php: Error thrown
* line 297 of /admin/tool/usertours/classes/step.php: call to tool_usertours\target::get_target_instance()
* line 77 of /admin/tool/usertours/classes/output/step.php: call to tool_usertours\step->get_target()
* line 66 of /admin/tool/usertours/classes/output/tour.php: call to tool_usertours\output\step->export_for_template()
* line 80 of /admin/tool/usertours/classes/external/tour.php: call to tool_usertours\output\tour->export_for_template()
* line 244 of /lib/externallib.php: call to tool_usertours\external\tour::fetch_and_start_tour()
* line 79 of /lib/ajax/service.php: call to external_api::call_external_function()
I guess it may not be that big a deal in the grand scheme of things, but I'd really like to understand the issue and ideally fix it, then I can feel confident about Moodle. If anyone has any ideas, I'd be eternally grateful....