Proper use of Get-Childitem with a PowerShell Foreach loop - powershell

Question

I want to take a text file with usernames, and then search a specific location (UNC path) and return any matches found, outputting them to a log file:
#Searches target folder for all folders matching input
$start = (get-date -uformat "%Y-%m-%d_%H%M")
$defaultLogFileName = "folder_matcher" + $start + ".log"
#Log file header
$header = #()
$header += "=============================="
$header += "Search results"
$header += ""
$header += "Execution Start: " + (get-date -uformat "%Y-%m-%d %H%M") + ""
$header += ""
#Get the logfile location
do
{
$logdir = Read-Host "Enter log directory (or press Enter for default c:\)"
if ($logdir -eq "")
{$logfile = ("c:\" + $defaultLogFileName); break;}
if(Test-Path $logdir)
{$logfile += ($logdir + "\" + $defaultLogFileName); break;}
Write-Host -ForegroundColor Red "Directory does not exist"
} while (!(Test-Path $logfile))
$SourceFile = Read-Host "Enter file path"
$SearchValue = Read-host "Enter target directory to sweep"
$header | Out-File -FilePath $logfile
foreach($user in $sourcefile){
Get-ChildItem $SearchValue -filter $user | Out-file -Append -FilePath $logfile
}
When I attempt to use Get-ChildItem in this loop, this is the result:
Get-ChildItem : Second path fragment must not be a drive or UNC name.
Parameter name: path2
At \\erebus\erebus_users$\rraymond1\ps\searchandmatch.ps1:32 char:14
+ Get-ChildItem <<<< $SearchValue -filter $user | Out-file -Append -FilePath $logfile
+ CategoryInfo : InvalidArgument: (\\harvard\tsprofiles$:String)
[Get-ChildItem], ArgumentException + FullyQualifiedErrorId DirArgumentError,Microsoft.PowerShell.Commands.GetChildItemCommand
I have tried replacing the variables with fixed values (even mapping the UNC path locally and trying to run it that way), and the problem remains. How can I fix it?