From time to time Anti-virus software and some browser updates may treat some websites as potential security risks or adware when previously there was no problem. If you get a security warning when trying to access the builder you may need to configure your firewall settings and add http://builder.mp-app.com to the list of trusted sites.
You may also find it beneficial to switch to using https://builder.mp-app.com version of the MyPhotoApp builder.
This recently happened with the latest version of Kaspersky Antivirus treating MyPhotoApp as an adware software and depending on your security settings it may have blocked the MPA builder.
If you are running Kaspersky antivirus and have difficulty accessing the builder the solution is to add
http://builder.mp-app.com as a trusted site in Kaspersky firewall settings.