I saw your print-outs and they had strange reference numbers. With this backup, I could still not reproduce your errors by entering a service item and a new customer. When Printing your attached invoice and purchase order I could still not reproduce this error for both documents.
There must be some mistake in your code somewhere.
If you want to try the source, that I am using, you can download it from our HG Repository:
https:/sourceforge.net/p/frontaccounting/mercurial/ci/default/tree/
Press the 'Download Snapshot' link up right. After a while a zip file is created that you can download.
Try to make a new installation with this new zip file and restore your attached sql backup file on your earlier Company.
When I tested, I changed the prefix in the file, fl_ , to 0_ to make my Life a Little easier.
Hopefully it is working for you with this new installation.
/Joe