file_get_contents(module) error at /admin/content/node only

When I enter /admin/content/node for the first time (after clearing cache), the following error is logged:
file_get_contents(module) [<a href='function.file-get-contents'>function.file-get-contents</a>]: failed to open stream: No such file or directory in <basepath>/includes/locale.inc on line 1685.

Besides, also a "file not found" warning appears (location /module?x, where x is different character like 3, O, ...) after that.

This warning is logged always after a /admin/content/node visit, while the error as above is logged only occasionally (I believe only when cache is emptied or needs to be refreshed...).

I am attaching also the extract from generated page source:

<link type="text/css" rel="stylesheet" media="print" href="/sites/all/themes/my_theme/print.css?O" />
    <script type="text/javascript" src="/misc/jquery.js?O"></script>
<script type="text/javascript" src="/misc/drupal.js?O"></script>

<script type="text/javascript" src="/misc/form.js?O"></script>
<script type="text/javascript" src="/files/languages/sl_9832859ab82db780660bbde670959c93.js?O"></script>
<script type="text/javascript" src="/sites/all/modules/jstools/jstools.js?O"></script>
<script type="text/javascript" src="/sites/all/modules/collapsiblock/collapsiblock.js?O"></script>
<script type="text/javascript" src="/sites/all/modules/jstools/jquery.cookie.js?O"></script>
<script type="text/javascript" src="/sites/all/modules/spamspan/spamspan.compressed.js?O"></script>
<script type="text/javascript" src="/misc/tableselect.js?O"></script>
<script type="text/javascript" src="/misc/tableheader.js?O"></script>
<script type="text/javascript" src="/module?O"></script>
<script type="text/javascript">
<!--//--><![CDATA[//><!--
jQuery.extend(Drupal.settings, { "basePath": "/", "jstools": { "cleanurls": true, "basePath": "/" }, "collapsiblock": { "blocks": { "block-adminblock-0": "2", "block-user-2": "2", "block-user-0": "1", "block-statistics_trends-0": "2", "block-system-0": "1" }, "default_state": "1", "slide_type": "1", "slide_speed": "200" }, "spamspan": { "m": "spamspan", "u": "u", "d": "d", "t": "t" } });
//--><!]]>
</script>
    <!--[if lt IE 7]>
      <link type="text/css" rel="stylesheet" media="all" href="/sites/all/themes/my_theme/fix-ie.css" />    <![endif]-->
  </head>

I believe the line:
<script type="text/javascript" src="/module?O"></script>
is the one creating problems.

It is interesting I've noticed this error only on this URL - everything else seems to work just fine. I've noticed this error now, after upgrading to Drupal 9, but it might be present for a while as I don't visit this URL often...

I have no clue how it is generated though. I've tried to set some drupal_set_message/var_export clauses all around to print out some variables, but without success. When javascript part of headers is generated, everything seems to be normal (as much as I was able to read out).

When I gave up and tryed to disable modules one by one, I've realized that after disabling Weight module, all problems disappear. So, I blame this module for the issue I am describing (I have everything else back as before, just without Weight module, and everything seems to be OK...)

Any useful tips (if nothing else, how to successfully debug the code) are highly appreciated.