error in edit_block_popup.php

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Hi I am experiencing a following problem with concrete 5.4.0.5:

As administrator I did the following:
1. Created new group (eg "newgroup")
2. Created a new user (eg "newuser") and assigned him to the newgroup
3. Went to the front page, clicked edit, clicked permissions and assigned read/write permissions to newgroup
4. Logged out

Then I've logged in with newuser, went to a page, clicked "add to main", => Add Block => Content

After typing anything in the content and clicking save/add I get the following error:

<b>Fatal error</b>
: Call to undefined function _() in
<b>/var/www/html/concrete/tools/edit_block_popup.php</b>
on line
<b>22</b>

However if I try to add a block as an admin, it works fine.

Any ideas?