Contao: submitOnChange gives blank page

[adojck]

Affected version
Contao 4.6.9

Description
Where submitOnChange is used, after selecting option, the page refreshes into blank page (no errors logged, no messages, no HTML in page). Happens with our own bundle, and with standard bundles too.

How to reproduce
1) Go to: Themes -> Front-End Modules -> New Module and then change Module Type, after page reloads it stays blank
2) Go to: Articles -> Edit any article -> New Element and then change Element Type, after page reloads it stays blank

Notes
PHP versions tested: 7.1 and 7.2
No errors in logs... No content in empty page.
Same problem with all browsers (_Firefox, Chrome, Opera_), so I think it's browser independent.

[fritzmg]

I cannot confirm that (Contao 4.6.9, PHP 7.1).

[adojck]

Found the reason of this problem.

In logs showed up this:
[warn] PHP Warning: Cannot modify header information - headers already sent by [...]

Headers was already sent by template file which was saved as UTF-8 with BOM.

[adojck]

We found all files which has UTF-8 BOM with this command:
grep -rlIR $'\xEF\xBB\xBF'
Source: https://www.dynobin.com/linux-remove-byte-order-mark-bom-from-files/