Problem: My website built on T3 Framework and t3_bs3_blank crashed and failed in Developer mode. Compile LESS to CSS function also failed.
Cause: I updated the t3_bs3_blank template from older J3 version to newest November 2023 version after I updated J3 to J4.
Solution: use Akeeba backup to restore website to J3 state, then update T3 and t3_bs3_blank to most recent versions, then update from J3 to J4.
Problem solved. So far.
Thanks.