WackoWiki

View source for Re: Once again about an empty screen | error source

The culprit: U_ILLEGAL_ARGUMENT_ERROR
%%
Fatal error:  Uncaught IntlException: datefmt_create: invalid locale: U_ILLEGAL_ARGUMENT_ERROR in D:\xampp\htdocs\wikieng\class\wacko.php:536
Stack trace:
#0 D:\xampp\htdocs\wikieng\class\wacko.php(536): IntlDateFormatter->__construct('C', 0, 0, 'UTC', NULL, 'dd.MM.yyyy HH:m...')
%%
((https://github.com/php/php-src/issues/12561 IntlDateFormatter constructor throws an exception for unknown locales since 8.1.25))

Somehow your installation passes ##C## (un'C'onfigured) as local, but it should be something like ##en_US.utf8##. 

((https://www.php.net/manual/en/migration80.incompatible.php#migration80.incompatible.standard PHP 8.0: Backward Incompatible Changes))
%%The default locale on startup is now always "C". No locales are inherited from the environment by default. Previously, LC_ALL was set to "C", while LC_CTYPE was inherited from the environment. However, some functions did not respect the inherited locale without an explicit setlocale() call. An explicit setlocale() call is now always required if a locale component should be changed from the default.%%

Its a ##setlocale()## issue. I assume the value is missing due to an old or outdate cached system default.

In Wacko class ##set_language()##:
##$this->lang['locale'] = setlocale(LC_CTYPE, 0);##

If you modify the lines in ##/etc/apache2/envvars## as follows ...
%%(ini)
## The locale used by some modules like mod_dav
# export LANG=C
## Uncomment the following line to use the system default locale instead:
. /etc/default/locale
%%
and restart Apache. Then Apache will use the locale and language settings from your operating system.
https://stackoverflow.com/questions/65502589/php-where-does-setlocales-value-come-from

Please try to delete the config cache, **restart** your XAMPP so it will take the new value. Guess this should do it, but I'm not sure - having no Windows OS at hand for testing.

On my Linux machine with XAMPP all works as expected.

For a possible fix see comment below.