Strange I don't have any error regarding the filter on my side :(
Anyway you can try it like that :
$CSV = Import-Csv -Path Example_csv.csv -Delimiter ";"
foreach($line in $CSV) {
$ADUser = Get-ADUser -identity $line.sAMAccountName
if($ADUser -ne $null) {
Set-ADUser $ADUser -Description $line.Description
}
}