Installing Invoiceplane in a subfolder - "404 Error"

Ok I tested it and indeed there is an issue with the provided standard htaccess.

Let me assume:

1. you have enabled the “REMOVE_INDEXPHP” feature?
2. you have installed IPv1 in a subdirectory? (ok this is obviously)

Could you tell me the exact subdirectory in which you installed InvoicePlane? Then I will provide you the corrected htaccess.