changing from php7.4 to php8, unable to start joomla

[taksejimo]

Changing from php 7.4 to php 8, the site got the error "count(): Argument #1 ($value) must be of type Countable|array, int given" at start up time.
The site works normally with php7.4. Joomla version is 3.10.10.
Please advice me what is wrong , how to fix it.

Thanks in advance, T.Sejimo

[gws]

One of your extensions/templates is not compatible with php 8.

[AMurray]

turn on the debug function in joomla, and in your web-host settings. (php.ini file or other method), which will help trace what is causing the problem.

Put PHP back to 7.4 first, make the above change, then swtich to PHP 8 and see what the debug comes back with; it should identify the actual script causing the error.

At the same time, providing the FPA report may help us identify the extension.
Refer the red box top of screen: Forum Post Assistant(FPA): https://forumpostassistant.github.io/docs/

[sozzled]

I agree with @AMurray. PHP 8 flags these "countable" issues as fatal errors (while PHP 7 flags them as warnings) and reverting to PHP 7.4 should allow you to work [relatively] normally. The most likely cause is a third-party extension that's not 100% compatible with PHP 7.2+ and, enabling J! debug mode (setting the error reporting level to "maximum" should identify the source of the problem. When you've discovered the source of the problem, contact the author of the extension to find if there's an update that fixes this "countable" error.