Check a Lists BrowserFileHandling. --> Set it to Strict or Permissive
Add-PSSnapin -Name Microsoft.SharePoint.PowerShell –erroraction SilentlyContinue
$problem_web = "https://mysharepoint/"
$problem_list = "Tasks"
$set_bfh_to = "strict"
#$set_bfh_to = "permissive"
$web=get-spweb $problem_web
$list=$web.Lists[$problem_list]
write-host "BrowserFileHandling Setting:"$list.BrowserFileHandling -ForegroundColor GREEN
$web=get-spweb $problem_web
$list=$web.Lists[$problem_list]
$bfh_old = $list.BrowserFileHandling
$bfh_new = $list.BrowserFileHandling = $set_bfh_to
write-host "BrowserFileHandling Setting set from $bfh_old to $bfh_new."-ForegroundColor GREEN
$list.Update()