Thanks Kovid & Chaley for all your quick support.....Finally it got resolved
Actually, what I did before as per Chaley's post recommendation I have only deleted the CACHE Files inside the Directory
calibrePortable\Calibre Settings\caches\gui-thumbnail-cache* & it was empty folder till now, later after Kovid's msg I Deleted the whole folder & it got recreated ...which indeed brought all the thumbnails back.
Please find the updated Debug logs attached below
One Issue I always face, whenever new update comes for Calibre & I try to install the Portable update in the specific folder....my KASPERSKY Antivirus try to block this, I need keep Kaspersky in PAUSE mode & then do the update....why this happens