google.script.run fails in case of file input

I have a google apps script bounded to a spreadsheet, and I created a html form to upload csv and excel files to process them and load the data to the spreadsheet. Everything worked fine in the last 2 years, but since March 2020 the file uploading fails.

When I submit the upload form, I got the following message:

We're sorry, a server error occurred. Please wait a bit and try again.

I checked and the the server-side function is not even called. If I remove the file input field from the form, then everything works fine. I mean the server-side function is called, but of course it hasn't got the file blob...

I didn't modified the script in the last half year, so I thought something changed in the google.script.run function, but I haven't found anything about that.

I also thought that maybe the file has problems, so I tried other files, which I already uploaded, but I got the same error.

Here is my form:

 <form id="uploadForm" onsubmit="uploadCsvClient(this)">
     <input name="fileToUpload" type="file"/>
     <input type="submit" value="Upload"/>
 </form>

The client side script:

function uploadCsvClient(formObject) {        
           google.script.run.withSuccessHandler(uploadSuccess)
                    .withFailureHandler(onFailure)
                    .uploadCsv(formObject);
      }

Do you know about that something is changed in GAS? What could be the problem? I'm totally lost, so I would really appreciate any help...