First, have you checked that the USB Mode hasn’t somehow been changed? Settings/System Settings/USB Mode. For direct drag-and-drop, it should be MSC. (MTP is for use with Windows Media Player, Auto Detect just makes trouble.)
It is fully possible that the Sansa is just old and its memory is going bad.
But…what operating system are you using? (Dolphin is cross-platform.)
If it’s not Windows, can you borrow a Windows computer and see if you can connect?
Go to Settings/System Settings/USB Mode and make it MSC. Connect to a Windows PC and see if you can just drag-and-drop something from a Windows folder onto the unit. That would help to figure out if it’s a problem with Dolphin or with the Sansa.
You could also run Windows’ chkdsk to try and fix memory errors. Open Computer or My Computer in Windows, find the Sansa Clip among the drives, right-click, Properties/Tools/Error-Checking and run it.
If you’re still having trouble you could always nuke the Sansa and start over, which can fix random weirdnesses. But that would erase everything you’d put on, so you would need to have copies. Format the Sansa via Settings/System Settings/Format. After formatting, check to see that it’s still USB Mode MSC.