Laravel 5.3. Certain word causes method not found exception when posted

I encountered laravel method not found exception while trying to post this sentence to controller

5-6. It's ___ that penicillin, a ____ for various infections, was discovered by accident after some mold grew in a laboratory dish.

I tried removing parts of the sentence and found out that whenever the word "penicillin" included, the error shows up.

What could be causing this issue ?

Here is code of my form. Im using tinymce plugin with laravel 5.3

<form class="form-horizontal" role="form" method="POST" action="">
        
        <div class="row">

          <div class="col-xs-12 col-md-12">

              <div class="">
                  <label for="question" class="control-label">Question</label>
                  <textarea maxlength="50000" rows="10" id="question" type="text-area" class="form-control" name="question" value="" required autofocus>  </textarea>

                  @if ($errors->has('question'))
                     <span class="help-block">
                       <strong></strong>
                     </span>
                  @endif

            </div>
            <div class="spacer-s"></div>
          </div>
       </div>

       <div class="row">
           <div class="col-xs-12 col-md-12">
             <div class="action-wrapper">
                <button type="submit" class="btn btn-primary">Add</button>
             </div>
           </div>
       </div>
</form>

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