• windows 10

• FF 58.0b11 (64-bit) uBo 1.14.21rc4
  or

• chrome Version 64.0.3282.24 (Official Build) beta (64-bit) uBo dev build 1.14.21.104

• go to google.com

• open dashboard, logger

• refresh page

• choose some element to block for example:
  notifications.google.com

• Click on the 4th cell of a log entry => to create a static filter

• create the filter => ||notifications.google.com

Now refresh the page, go into "my filters" tab, delete the filter.
Repeat exactly the same steps above.
When the filter was created exactly as in the first step => the "Create" button is disabled.

It does not matter which is the test page.