we have seen this happening when people are using tools like Dropbox or Box or Google Drive or One Drive and have the active synching going on in background. Those systems when they grab a file to sync from your hard drive lock the file in some way while they sync and that triggers this issue in TBC. If you turn off the synching while working on TBC this problem should go away.
When TBC imports images or pdf pages or point clouds, the data is not actually stored in the VCE file, it is stored in the project folder and referenced into the project. If you start a new project you can also try saving the project prior to importing these types of files so that the project folder is already created, and then import the PDF pages and reference them, that way they have a "home folder" where they can be located and that may also mitigate the issue you are seeing. If that fails then try turning off sync to the file system you are using and see if that fixes the issue.
If not let me know and we can take a look in a GOTO Meeting this week
If the images don't show up, try Support, Options, Images and set to No Tiling and see if they show up then
You can try resetting to Tiled images once they become visible
Let me know if this helps - if not call me on +1 720 323 0481
Alan