Without any info about your setup it’s hard to tell.
But I guess it mostly is due to the .htaccess. Did you set it up right and can you provide some logs from your webserver?
## Run InvoicePlane in a subfolder
## If you are using a subfolder please remove the hash in front of the line ‘#RewriteBase /subfolder’
## and replace ‘subfolder’ with the actual name of your folder, i.e. ‘RewriteBase /invoices’
<IfModule mod_rewrite.c>
RewriteEngine On
#RewriteBase /subfolder
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
## Remove the hash in front of the following two lines if you want to force HTTPS
#RewriteCond %{HTTPS} off
#RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L]
</IfModule>