Thank you!
I didn’t actually needed the SFTP after all. So, the issue is caused by the page minification – and the issue was not present to admin accounts, as it is probably turned off in for administrators. I could not find which plugin is doing the page minification, but it actually adds an extra character to the JSON string that is used in the plugin init. I don’t know why it is doing that, it probably recognizes some sort of a pattern incorrectly.
Luckily there is an option to change the loading method within the plugin. I have changed that here: https://i.imgur.com/Uf53bjH.png
It should be okay now.
I still recommend turning off/reconfiguring the HTML minification or switching to a different solution if possible.