Problem 1
Seeing this:

The reason is the filename including the folders are too long.

Solution: Rename the file with a shorter name and that includes the folder names. You may get errors so start shortening the folder names.

Problem 2

Seeing a weird thumbnail and when you click on it it says "an internal error occured" then try this:

Solution: Go to Database | Quarantine Files & check to see if they're in the there. If it is there, click on the filename, and select remove. This will remove it from the quarantine list.

Quarantine files are files that ACDSee at some point had an error opening and it placed it in the Quarantine files. (Think of quarantine as files that are error'ing.) It may be that the file is now okay but ACDSee won't try open files that are in this list. Once you remove it, ACDSee will try open the file. If it succeeds you are fine and if there is still a problem it will be put in the quarantine list. Note: ACDSee's Quarantine files have nothing to do with viruses.