eBay tech support isn't here, and eBay isn't reading this forum (unless one of the blues gets bored or curious). It's just us other users here.
My eBay messaging is working fine, and nobody else has reported that issue that I've seen. (which leans toward it being a you problem not a widespread eBay problem)
If you are using the mobile app or the mobile version of the website on a device you need to go ask in the mobile forums - nobody here will know anything about mobile except that the app pretty much sucks and has problems constantly.
https://community.ebay.com/t5/Mobile-App-Android/bd-p/androidapp
https://community.ebay.com/t5/Mobile-App-iOS/bd-p/iosapp
I don't know if there are different eBay messaging inbox pages, but you might try going to messaging inbox directly here: https://mesg.ebay.com/mesgweb/ViewMessages/0/All
If that fails the same way, try clearing browser cache and cookies and restarting the browser (Thursday night was eBay maintenance night and they may have changed something, but you have the old stale files stuck in your browser cache). If that still doesn't do the trick, try another browser.
