This is a typical error.log entry in a Linux, Engine X, MariaDB, PHP (LEMP) occurring multiple times in referenced version:

2021/01/07 01:23:50 [error] 2563#2563: *7876 FastCGI sent in stderr: "PHP message: PHP Notice: Undefined index: image_alt in /path/to/my/b2evolution/inc/_core/_misc.funcs.php on line 9309" while reading upstream, client: abc.st.uv.xyz, server: myb2evolution.blog, request: "GET /home/ HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.4-fpm.sock:", host: "myb2evolution.blog"

Filed here for posterity as the b2evo devs' silence around here is deafening. ;-)