Unable to load version 4.5 on MAC

I was using version 4.5 on a MAC (OS X El Capitan).  Everything seemed to work the first few times I tried to borrow a book, but now when I try to open the ACSM from my public library, I get the following message:

Unable to download

Error getting License. License Server Communication Problem:

E_IO_CANNOT_OPEN

I have no problems on my PC (version 1.7.2) but this is not available for MACs.  I found other posts which indicated the solution was to use version 1.7.2 of Digital Editions.  I uninstalled version 4.5 and tried to install versions 2.0.1 and 3.0 but the installations failed.  I went back and tried to install version 4.5 and that has now failed.