Have you upgrade Filemaker Go to the most current version on the iOS 8 devices? This will fix most issues.
I have upgraded and have actually done clean installs of the latest version available in the app store. No avail.
I created a sample app with a button with a script "Insert Signature from device" and I also clicked "Run with full access privileges". I then upload to my iPad with iOS8, click the button and insert signature then I clicked button and replaced with new signature without error.
1) My be a privileges problem, seem that iOS8 is more restrictive. Make sure users have correct privileges or click to have the script to run with full privileges as I did in my sample.
2) If you continue to have problems, then please give more information. Version of Filemaker 12 or 13 that db was created, Type device iPad, iPhone, iPod, copy of your script.
Sorry, reread your post and I will also test on my iPhone and let you know if there is any problems.
I tested my sample app on my iPhone 5 and I still had no problems.