Transfer of acsm file to ADE

I download ACSM files from two different lending libraries to my computer.  I transfer the  acsm files from my downloads folder to the same ADE installed on my computer.  The acsm files from one library (Open Library) are  transferred without difficulty.  The acsm files from the other library (my public library running on OverDrive) are not accepted by ADE; an error message appears saying "Server's communications problem: E ADEPT_IO.  This did not occur until about a year ago.  Until then ADE handled the files from the public library wthout any problem.  The only 'external' difference  between the files is that he pulic library files are named for the book title with a number, while the other files are all named URLLink. How come ADE accepts acsm files from one source and not from an another?