Issue: Unable to upload attachment

I am currently running into issues with reuploading my latest update to Sigma Emerald (uploading results in "upload of file failed" message). Continued attempts to upload eventually results in the following screen regarding a missing security token:

Attachment test

Try logging out or clearing your browser cache / cookies / website data for PC. Also try a different style?

Second attachment test

Nothing doing :/ I've logged out, cleared my cache, and tried using different themes

Attempting an attachment test leads to this:

Can you check what other browsers do?

Firefox and MS Edge yield the same result :/