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()