I am running photoshop CC 2018 19.1.2 on Windows 10 Pro, version 1709, build 16299.334.
The problem is resolved.
I did some more searching and found a similar problem: Unable to Import Image Sequence into Premiere Pro CC 2015
It appears that the Premiere and Photoshop image sequence import is very sensitive to filename format. The video file that I opened in Photoshop was named VID_20180401_115900678.mp4. When I exported the image sequence, Photoshop created the image sequence files with filenames of the form:
The last 4 digits of the filenames are the sequence numbers and were created by the Photoshop export process.
When I attempted to import this sequence, I got the "file name was not valid" error.
Based on some of the comments in Unable to Import Image Sequence into Premiere Pro CC 2015 , it appears that Photoshop is sensitive to the exact format of the filename. I suspected that the numbers in my video filename were causing the problem so I renamed the video file to VID_20180401_115900678-.mp4 (note the trailing '-' in the filename) and repeated the entire process of opening the video file, exporting the image sequence, and then opening the generated image sequence.
This time, when I exported the image sequence, Photoshop created the image sequence files with filenames of the form:
Note the '-' character that separates the numbers in the filename from the image sequence numbers.
The open image sequence worked with this set of files.
It appears that Photoshop expects at least one non-numeric character to separate the sequence number from the rest of the filename.
Adobe should document this behavior in the instructions Import video files and image sequences in Adobe Photoshop so that users are aware of the exact naming requirements for image sequences.