HTTP/404 on Special:Upload when using POST

Hello list,

for some reason I'm no longer able to use the upload function on my little 
Mediawiki installation. I haven't uploaded things a while, and recently 
updated to 1.43.1, and everything else appears to work quite well, except 
uploads. So, I navigate to Special:Upload and in the Apache web server log 
I can see (shortened, w/o IP adresses and user information):

 "GET /wiki/Special:Upload HTTP/2.0" 200 5795 "-" "Mozilla...

Great, HTTP/200 looks good, and the "Upload file" page is being displayed, 
and I click on "choose file" and select a .gif and the .gif is displayed 
now displayed as a little preview in the "Source file" part of the Upload 
page. But then I hit the "Upload file" at the bottom of the page and get a 
"Not Found" message in my browser window. And the same Apache web server 
log now has:

 "POST /wiki/Special:Upload HTTP/2.0" 404 291 "https://foo.example.org/wiki/Special:Upload" "Mozilla...

So, GET works just fine, but POST produces a HTTP/404?

I've disabled mod_security, disabled all CSP and X-XSS-Protection etc. 
headers too, but I'm at a loss on how to debug this further.

In my virtual host configuration of the Apache web server, the following 
is included:

  RewriteRule ^/wiki/(.+)?$  /mediawiki/index.php [L]

This worked quite well over the years, and I *was* able to upload stuff 
before, and with that configuration too.

Attached below is my (shortened) LocalSettings.php (w/o some database 
secrets and names).

Has anybody seen something like this before, and maybe even remembers 
where to look to debug this?

Thank you,
Christian.

===== LocalSettings.php
$wgSitename		= "foo";
$wgScriptPath		= "/mediawiki";		# real path
$wgArticlePath		= "/wiki/$1";		# virtual path
$wgUsePathInfo		= true;			# ?
$wgServer		= "https://foo.example.org";
$wgEnotifUserTalk	= true;		# UPO
$wgEnotifWatchlist	= true;		# UPO
$wgEmailAuthentication	= true;
$wgDBtype		= "mysql";
$wgDBserver		= "127.0.0.1";
$wgDBname		= "foo";
$wgDBuser		= "foo";
$wgDBpassword		= "xxxx";
$wgDBprefix		= "";
$wgMainCacheType		= CACHE_ACCEL;
$wgCachePages			= true;
$wgParserCacheExpireTime	= 604800;
$wgUseFileCache			= true;
$wgEnableSidebarCache		= true;
$wgShowIPinHeader		= false;
$wgEnableUploads		= true;
$wgUseImageMagick		= true;
$wgImageMagickConvertCommand 	= "/usr/bin/convert";
$wgShellLocale			= "en_US.utf8";
$wgCacheDirectory		= "$IP/cache";
$wgFileCacheDirectory		= "$wgCacheDirectory/html";
$wgLanguageCode		= "en";
$wgSecretKey = "xxxx";
$wgUpgradeKey		= "xxx";
$wgDefaultSkin		= "Vector";
$wgDiff3		= "/usr/bin/diff3";
$wgGroupPermissions['*']['read']		= true; 
$wgGroupPermissions['*']['edit']		= false;
$wgGroupPermissions['*']['createaccount']	= false;
$wgNoFollowLinks	= false;
$wgNamespacesWithSubpages[NS_MAIN]	= true;
$wgAllowExternalImages			= true;
$wgAllowCopyUploads			= true;
$wgCopyUploadsFromSpecialUpload		= true;
$wgFileExtensions		= array_merge($wgFileExtensions, array( 'pdf', 'txt', 'gz', 'bz2', 'deb', 'zip', 'svg', 'diff', 'json' ));
$wgVerifyMimeType		= false;
$wgDisableUploadScriptChecks	= true;
$wgExtraNamespaces = array(
$wgNamespacesWithSubpages[NS_FOO]		= true;
$wgNamespacesWithSubpages[NS_FOO]	= true;
$wgShowExceptionDetails	= true;
$wgDevelopmentWarnings	= true;
$wgSitemapNamespaces		= array( 0, 4, 12 );
$wgAllowTitlesInSVG		= true;
$wgSVGConverter			= 'ImageMagick';
wfLoadSkin('Vector');
wfLoadSkin('MinervaNeue');
wfLoadSkin('Timeless');

-- 
BOFH excuse #308:

CD-ROM server needs recalibration