markus99's question about the error was answered in another post.
"The file is to large..." error:
vi-solutions.de/en/forum-index/17-joomla...-to-large-error#8872
Citation:
Hi Marcus 99,
in your forms configuartion there is an option, where you select the directory, where the uploaded files should be stored.
This option was set on the Joomla 3 website.
The value was imported to the Joomla 4 website.
Actually, it looks to me, as if this directory does not exist on your Joomla 4 website.
Therefor you got the error message "Upload directory (advanced options) doesn't exist", after the import.
So please go the the form configuration.
Select the tab "Advanced".
Almost at the bottom, there is a section "File-uploads".
Please make sure, that you enter the name of an existing directory in the option "Uploaded Files Directory".
Then save the form configuration.
After that, uploading files with the form in the frontend should work.
I'm sorry, that the error message "Max file size is 0 kb" is somewhat misleading.
Kind Regards,
Aicha