[2.6.14] TCPDF error: unable to write cache file

[iWim]

Hallo,

When I want to download an invoice from the VM order list I get a page with the line

TCPDF ERROR: Unable to write to cache file: /tmp/font_123ABC (123ABC is random, changes every time I click the download pdf image)

The page is located at index.php?option=com_virtuemart&view=invoice&layout=invoice&format=pdf&tmpl=component&virtuemart_order_id=116&order_number=********&order_pass=p_*****

/tmp/ is writable (755).
The invoice is created, I can find it through FTP.
Printing the invoice works fine.

Specs:
J! 2.5.28
VM 2.6.14

I have two other shops running on J! 2.5.27/VM 2.6.12.2 that does not have this issue.

The error is also displayed with invoices created before the update to 2.6.14.

Any idea what this could be?
More importantly how can I solve this?

Thank you.
Wim