Steps to reproduce:
1. Go to some quiz -> results -> grades.
2. On 'Download table data as' field select xlsx or ods.
3. press download button

Expected: valid xlsx or ods file downloaded.

Actual result: File not found.

Reproduces always on all quizes.

Error log:
2016/10/11 18:32:43 [error] 632#0: *323 FastCGI sent in stderr: "PHP message: Default exception handler: Исключение - The sheet's name is invalid. It did not meet at least one of these requirements:

• It should not be blank
• It should not exceed 31 characters
• It should not contain these characters: \ / ? * : [ or ]
• It should be unique Debug:
  Error code: generalexceptionmessage

• line 84 of /lib/spout/src/Spout/Writer/Common/Sheet.php: Box\Spout\Writer\Exception\InvalidSheetNameException thrown
• line 56 of /lib/classes/dataformat/spout_base.php: call to Box\Spout\Writer\Common\Sheet->setName()
• line 1743 of /lib/tablelib.php: call to core\dataformat\spout_base->send_http_headers()
• line 1040 of /lib/tablelib.php: call to table_dataformat_export_format->start_table()
• line 746 of /lib/tablelib.php: call to flexible_table->start_output()
• line 696 of /lib/tablelib.php: call to flexible_table->add_data()
• line 1495 of /lib/tablelib.php: call to flexible_table->add_data_keyed()
• line 66 of /mod/quiz/report/overview/overview_table.php: call to tabl" while reading response header from upstream, client: 10.130.0.76, server: m-mdl0.szgmu.local, request: "GET /mod/quiz/report.php?sesskey=JUm9nHPn35&download=excel&id=6522&mode=overview&attempts=enrolled_with&onlygraded=&group=0&onlyregraded=&slotmarks=0 HTTP/1.1", upstream: "fastcgi://unix:/run/php5-fpm.sock:", host: "m-mdl0.szgmu.local", referrer: "http://m-mdl0.szgmu.local/mod/quiz/report.php?id=6522&mode=overview"

CSV, JSON and HTML files downloads good.