Image thumbnail in grid displays inconsistency

[BigBlockStudios]

I am having a very strange problem with image thumbs in a nested grid. Sometimes they will display, other times not. Using either the Upload Files or Load from media source - thy both work in the sense that they will upload the images correctly and create a record in my database. BUT it is not showing some thumbnails. i.e. this particular image: "FB_IMG_1502019735126.jpg" always displays a thumb, if I make a copy of it and rename it (FB_IMG_1502019735126_XXX.jpg) it will not display a thumb.

in the logs I get phpthumb errors.

(ERROR @ /var/www/vhosts/domain.com/core/model/phpthumb/phpthumb.class.php : 1686) PHP warning: getimagesize(/var/www/vhosts/domain.com/dev.domain.com/FB_IMG_150201973512XXX.jpg): failed to open stream: No such file or directory

(ERROR @ /var/www/vhosts/domain.com/core/model/phpthumb/modphpthumb.class.php : 121) phpThumb was unable to generate a thumbnail for: /var/www/vhosts/domain.com/core/cache/phpthumb/dev.domain.com__srca1dcd6fc83866af5776e62cd0ce811c3_parfe75230c38c1dd77cfb659e43ae25583_dat0.jpeg

(ERROR @ /var/www/vhosts/domain.com/core/model/phpthumb/modphpthumb.class.php : 133) Error outputting thumbnail:
OutputThumbnail() failed because !is_resource($this->gdimg_output) in file "phpthumb.class.php" on line 618

The actual path for that image should be:

/var/www/vhosts/domain.com/dev.domain.com/assets/projects/{id}/images/FB_IMG_150201973512XXX.jpg

So no differences in the database record between FB_IMG_150201973512XXX.jpg & FB_IMG_150201973512.jpg one will work and the other not (even though they are the same image!!) Most images I upload will not display a thumb, but there are 3 or 4 more that behave exactly like this one.

Any thoughts as to what is going on?

(while we are at it, is there any way to swap/change an image from the thumb in the grid?)

[BigBlockStudios]

never found out why, but this just "started working" no explanation?

[Bruno17]

can't reproduce this issue