My fault, again. https://community.ipfire.org/t/proxy-problem-after-upgrade-to-core-155-the-proxy-server-is-refusing-connections/5020/24
On the second thought, I propose dropping this checkbox altogether. Squid's version information should not be exposed to anybody, and does not even matter to clients or non-authorized personnel.
https://patchwork.ipfire.org/project/ipfire/patch/b4f59ffb-9a04-938f-7c0e-5a39ded292e8@ipfire.org/
https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=13aeb192178b57bc1b14abc514a022ca89cc87bd
https://blog.ipfire.org/post/ipfire-2-27-core-update-161-available-for-testing
https://blog.ipfire.org/post/ipfire-2-27-core-update-161-released