Upgraded to LR 2015 and PS CC 2015. Now when I am in LR and edit in> PS 2015 it gives me a message "the file could not be edited because Adobe PS 2015 could not be launched. PS was already open so there was no issue of not being able to launch PS. The image opens after about a minute or two anyway. I also received "an internal error has occurred: agstoreprovidersession:getentitybyID:mucst be a number or string."
Thanks for any help on correcting this issue
Same happened to me.
I had to un-install ALL Adobe programs. Delete any old left-over folders with Adobe data. (Backup plug-in folders to re-install)
Then Re-install Photoshop-CC, Bridge, and Lightroom-CC from the Creative Cloud app.
I also ran CCleaner to tidy or delete left-over references to adobe programs in the Windows registry before the re-install.
The edit link LR to PS now working well.
Sorry to hear you had the same problem, and thank you for your solution. Adobe: this is a time consuming process which has never happened on previous updates so wondering why this is happening now and what are you doing to be sure this doesn't happen in the future?