When you open the “Network” tab in the developer tools of the browser and click on the requests that fail, is there an error message in the response of these requests?
What version of MIGX are you using?
Why did you upgrade to a 2-year-old version of MODX?
On the response tab I get “this request has no response data available”
The upgrade tile in the manager offerd me only one higher version 2.8.2 if I remember correctly. And upgrading to this fully broke the system, no login even possible. So I restored and returned to 2.8.1.
I now saw that generation 3 is offered too. Not sure if I should try to fix that first or just give 3 a shot.
If I recall correctly, MODX version 2.8.2 had some issue (that where fixed with version 2.8.3).
I would try updating to MODX 2.8.4.
The update to MODX 3 can still be challenging, depending on the extras you have installed. The chance that you create more problems with an update is quite high.
As you get a 500 error. Can you check the PHP error log file to see what error message is generated. (The location of the PHP error log depends on the server setup and is independent of MODX.)
The version of the MIGX extra might be relevant (and is probably more relevant than the MODX version).
For the 500 errors, please check the PHP error log to get the specific error messages.
For the MIGX problem: Can you post your MIGX configuration (right-click → Export/Import → copy the JSON field) and the schema, so I can test if I can reproduce the issue.