It seems that when I am editing control points that after adding perhaps 10 or 15 new sets of control points that the CPE window shows only one of the two images I am trying to connect. If I continue adding control points (you can sometime do this without actually seeing both images) then a GFL error is announced. This used to happen in APP1.4.2 too.
Quitting AP[PG] and restarting fixes this (for another 10 to 15 sets of CPs).
The way this error happens, consistently, is that I open a .pano and start editing bad links. I click on a red square, delete the existing CPs and then add my own by getting APG to look at two common areas. If there are more than 50 CPs I throw away the worst (I have options set to keep only the best 50 points). Eventually I click on a bad link and an unpainted image appears - perhaps it takes more like 20 links before it happens.
After adding control points blind to about three more pairs of images, neither image is painted. I can still add CPs, but on the next bad link I get the GFL error shown below. Generally I get the message shown, citing the first file/image and if I press on OK then I get the same message about the second file/image.