diff --git a/Services/Excel/classes/class.ilExcel.php b/Services/Excel/classes/class.ilExcel.php
index 6e13f94033a8..f7196a646e0a 100644
--- a/Services/Excel/classes/class.ilExcel.php
+++ b/Services/Excel/classes/class.ilExcel.php
@@ -37,6 +37,8 @@ class ilExcel
     public const FORMAT_BIFF = "Xls";
     protected string $format;
 
+    private string $noncharacters = '[\uFFFE-\uFFFF\uFDD0-\uFDEF]';
+
     protected ilLanguage $lng;
     protected Spreadsheet $workbook;
     protected string $type;
@@ -158,15 +160,23 @@ public function getSheetTitle(): string
      * Prepare value for cell
      * @param mixed $a_value
      * @return mixed
+     * @throws InvalidArgumentException
      */
     protected function prepareValue($a_value)
     {
         if (is_bool($a_value)) {
-            $a_value = $this->prepareBooleanValue($a_value);
-        } elseif ($a_value instanceof ilDateTime) {
-            $a_value = $this->prepareDateValue($a_value);
-        } elseif (is_string($a_value)) {
-            $a_value = $this->prepareString($a_value);
+            return $this->prepareBooleanValue($a_value);
+        }
+
+        if ($a_value instanceof ilDateTime) {
+            return $this->prepareDateValue($a_value);
+        }
+
+        if (is_string($a_value)) {
+            if (!mb_check_encoding($a_value, 'UTF-8')) {
+                throw new InvalidArgumentException('Invalid UTF-8 passed.');
+            }
+            return $this->prepareString($a_value);
         }
 
         return $a_value;
@@ -200,7 +210,9 @@ protected function prepareBooleanValue(bool $a_value): string
 
     protected function prepareString(string $a_value): string
     {
-        return strip_tags($a_value); // #14542
+        return $this->cleanupNonCharachters(
+            strip_tags($a_value)
+        ); // #14542
     }
 
     /**
@@ -555,4 +567,9 @@ public function mergeCells(string $coordinatesRange): void
     {
         $this->workbook->getActiveSheet()->mergeCells($coordinatesRange);
     }
+
+    private function cleanupNonCharachters(string $string): string
+    {
+        return mb_ereg_replace($this->noncharacters, '', $string);
+    }
 }