Steps to replicate:

1. Make sure you have debugging on (add following lines to you config.php)

$CFG->debug = (E_ALL | E_STRICT); // DEBUG_DEVELOPER
$CFG->debugdisplay = 1;
$CFG->debugstringids = 1; // Add strings=1 to url to get string ids.
$CFG->perfdebug = 15;
$CFG->debugpageinfo = 1;

2. Navigate to /reportbuilder/index.php and create a new report. Select Course categories  report source. Leave everything else as default. 

3. Make sure you have category column in the report. Click preview and then try to download the report as CSV. 

4. Observe following error dumped into a file.

<div class="notifytiny debuggingmessage" data-rel="debugging">Script /reportbuilder/download.php?sesskey=5F0rZQ4yMN&download=csv&id=2 mutated the session after it was closed: $SESSION->cachestore_session: default_session-core/coursecat<ul style="text-align: left" data-rel="backtrace"><li>line 791 of /lib/classes/session/manager.php: call to debugging()</li><li>line ? of unknownfile: call to core\session\manager::check_mutated_closed_session()</li><li>line 159 of /lib/classes/shutdown_manager.php: call to call_user_func_array()</li><li>line ? of unknownfile: call to core_shutdown_manager::shutdown_handler()</li></ul></div>