Check a Lists BrowserFileHandling. --> Set it to Strict or Permissive

Add-PSSnapin -Name Microsoft.SharePoint.PowerShell –erroraction SilentlyContinue

$problem_web = "http://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()