I encountered the exact same issue today. Both the option to export to FGDB via the AGOL GUI, as creating a replica via the REST endpoint fails. I have a featureservice with a few geometry layers, each with its own related standalone table. The standalone tables have attachments enabeled. I can export all of the layers, except for one standalone table (this one layer is also causing the 'create replica' to fail). I have been able to create replica's previously. The featureservice hoewever was taken offline of edits, and re-uploaded to AGOL.
Retrieving the attachments is now problematic since the create replica option is the only way to get the attachments tables to download.
I'd like to know if there is any way to check which attachments are corrupted, or if there is another workaround. I'm rather proficient with Python and JavaScript, so if anyone could share code/a workflow that would be amazing. I'd rather not go through the process described by scott.
I find it rather odd that creating featureservices with attachments, and collecting photos in the field with collector is so easy, while retrieving these attachments after field-collection has to be such a hassle (a "Download Attachments" GUI option would be brilliant).
Cheers