Actually, as a developer of lighttpd (a portmanteau of “light” and “httpd”), I can tell you that it is intentional that is an error. When changing the lighttpd configuration, it is strongly recommended to test the config using lighttpd -f /etc/lighttpd/lighttpd.conf -tt. If there is an error, fix the problem before continuing (before restarting lighttpd or before rebooting). If someone else with admin privileges – e.g. the automated package updater – makes config changes, it too should test the config and should rollback the changes if there is an error, and alert an admin for intervention. The lighttpd config changes made by the Turris Team in Turris OS 6 (treewide: use number prefixes for lighttpd configuration files (!913) · Merge requests · Turris / Turris OS / Turris OS packages · GitLab) unfortunately did not play nicely with /bandwidthd/ or rrdtool or possibly other custom configs. Breaking a working config is an error, and lighttpd flagged it as such.
Lesson learned: as @miska noted in Turris OS 6.0 aftermath - Community office - Turris forum the Turris Team renaming and reconfiguration of lighttpd include files could have been rolled out seperately and earlier than Turris OS 6.