"Base 64" error message when exporting PowerPoint file with embedded Excel tables (Studio 2019)

Hello Community!

I am working on a large PPT with embedded Excel tables, in Studio 2019. Here is what appeared on my screen when I tried to export the target file:

"The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or a non-white space character among the padding characters"

The French message was : "Échec de l'enregistrement du contenu cible : L'entrée n'est pas une chaîne Base 64 valide, car elle contient un caractère non-Base 64, plus deux caractères de remplissage ou un caractère non conforme parmi les caractères de remplissage."

Has anybody ever seen this message, and most importantly, would you have any leads as to why Studio is doing this and how to fix the problem?

I will be grateful for any help received. :)