Error Call to private method Bolt\Controller\Backend\ContentEditController::getFieldToUpdate()

Bolt 4.2.2
Local setup with SQLite, PHP 7.3.28 on a Mac
Trying to import a json file from a Bolt 3.2.latest site
bobdenotter/conimex 1.3.5
Will attach the imported json file and the 'target' Bolt 4.2 Contenttypes file, with only the relevant contenttype, the "institutions"

institutions.json.zip contenttypes_institutionsonly.yaml.zip

This is what happens at import:

evertalbers@MacBook-Pro-2 PCAF-Multilan % bin/console conimex:import importfiles/institutions.json 

 ! [NOTE] Importing Bolt database contents from: importfiles/institutions.json                                          

^ array:3 [
  "date" => "2021-08-20T14:48:36+00:00"
  "version" => "3.7.4"
  "platform" => "sqlite"
]
 // Importing ContentType institutions                                                                                  

   0/146 [>-------------------------------------------------]   0%
In Import.php line 223:

  Call to private method Bolt\Controller\Backend\ContentEditController::getFieldToUpdate() from context 'BobdenOtter\Conimex\Import'

Any help will be apreciated. If it helps to e.g. drop one or two fields, no problem.

bobdenotter / conimex

Error Call to private method Bolt\Controller\Backend\ContentEditController::getFieldToUpdate() #30