There have been a few posts about this error, so I figured I'd give one possible solution that I just found. I was trying to submit a proposal for a project, and kept getting the dreaded "Sorry, we were unable to save your application." error, in both Firefox and Chrome. So, knowing that some web systems have issues with form contents that include parenthesis or # signs, I removed both. Not sure which did it (I presume the parenthesis), but it let me submit my proposal after removing the parenthesis from my answers to the questions on the application.
Please reply and/or give me kudos if this solves your problem!
On a side note, I'm new to UpWork, and am already extremely disappointed with the (lack of) customer support. I already ugpraded my membership to be a subscribing member, but the UpWork support system always makes me submit my questions to the "community", rather than submitting a ticket an UpWork staff member can get directly. I have experienced both issues getting paid AND issues submitting proposals, both of which are urgent, and shouldn't require me to just submit it to the community forums. The issue getting paid STILL hasn't been resolved, after several days, and multiple promises from an UpWork moderator that a staff member would be in touch soon to assist me. The site and opportunities here look great, but I'm almost to the point of giving up on UpWork, just because it's so hard to get any kind of support from their staff. I hope they fix that soon.
Well issues around payment are very worrying. I'll be checking back to see how that panned out.
Basically found your post because I have the 'Sorry we were unable to save your application' error. Will go back and see if my covering letter had the offending parentheses etc you mention. Thank you for that!