I think I found the issue: I was trying to call eval_js while the AJAX request was running (synchronous request, async: false). Python was waiting for the AJAX request to timeout before running js_eval.

I have some code that runs something like this:

call = '({})({})'.format(code, args)
result = js.eval_js(call)

print(result)
return result

That return value then is sent as a response to an AJAX call from JavaScript to a Bottle server (which made the previous code run).
If I don't run eval_js and set result to anything - for example: result = 'hello' - the response is sent correctly, but if I use eval_js, firstly it freezes Pythonista for a few seconds, then it prints the correct result (from the eval_jscall), but the response is empty.

I guess the UI is breaking the connection, somehow, corrupting the response.