InvoicePlane 1.5.x Saving Settings Error

I also had this issue, I had a custom setup. Which the issue was related to this

Header edit Set-Cookie ^(.*)$ $1;HttpOnly;Secure
Had to # to fix the issue
Under Ubuntu 20.04 apache2 (apache2.config)
Header always append X-Frame-Options SAMEORIGIN
FileETag None
# Header unset Pragma
Header unset ETag
# Header set X-XSS-Protection: 1

Header edit Set-Cookie ^(.*)$ $1;HttpOnly;Secure