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Ok, I think this might be related to a missing folder for temporary files on the server. Will confirm in a bit.
Correct. Some days ago I apparently deleted the temporary folder needed by PHP to upload images and also used apparently by Mailpoet, or rather by admin-ajax.php of WordPress.
I found it helpful to find out which temporary folder is to be used by PHP:
<?php echo "\nTemp directory: ", sys_get_temp_dir(), "\n"; ?>
I got:
Temp directory: /var/www/virtual/mydomain.org/phptmp
So I created that folder on the server (htdocs is some sort of alias to the longer path above):
mkdir /htdocs/phptemp
chmod 755 /htdocs/phptemp(You can also do that using an S/FTP program with a GUI.)
I guess that can be modified using a php.ini file, but I went with this not create more havoc. I’m writing this here in case anyone experiences something similar and cannot seem to fix it using the instructions provided (because they are, in fact, incomplete).
Anyway, everything works again now. It had literally nothing to do with browser cache and that error message is a bit misleading.