Error message

• Warning: imagepng(/var/www/clients/client3/web8/web/sites/default/files/print_pdf/print_pdf_tcpdf/cache/__tcpdf_18626d02956f286716f223223685fb78_imgmask_alpha_2867bbe95fd4a87ae17f1f0c95bcdc65): failed to open stream: Operation not permitted in TCPDF->ImagePngAlpha() (line 7332 of /var/www/clients/client3/web8/web/sites/all/libraries/tcpdf/tcpdf.php).
• Warning: imagepng(/var/www/clients/client3/web8/web/sites/default/files/print_pdf/print_pdf_tcpdf/cache/__tcpdf_18626d02956f286716f223223685fb78_imgmask_plain_2867bbe95fd4a87ae17f1f0c95bcdc65): failed to open stream: Operation not permitted in TCPDF->ImagePngAlpha() (line 7337 of /var/www/clients/client3/web8/web/sites/all/libraries/tcpdf/tcpdf.php).
• Notice: Undefined offset: 1 in theme_print_url_list() (line 357 of /var/www/clients/client3/web8/web/sites/all/modules/print/print.pages.inc).
• Notice: Undefined offset: 1 in theme_print_url_list() (line 357 of /var/www/clients/client3/web8/web/sites/all/modules/print/print.pages.inc).
• Warning: imagepng(/var/www/clients/client3/web8/web/sites/default/files/print_pdf/print_pdf_tcpdf/cache/__tcpdf_19a8b59482820ada11e98a4735ed4585_imgmask_alpha_2867bbe95fd4a87ae17f1f0c95bcdc65): failed to open stream: Operation not permitted in TCPDF->ImagePngAlpha() (line 7332 of /var/www/clients/client3/web8/web/sites/all/libraries/tcpdf/tcpdf.php).
• Warning: imagepng(/var/www/clients/client3/web8/web/sites/default/files/print_pdf/print_pdf_tcpdf/cache/__tcpdf_19a8b59482820ada11e98a4735ed4585_imgmask_plain_2867bbe95fd4a87ae17f1f0c95bcdc65): failed to open stream: Operation not permitted in TCPDF->ImagePngAlpha() (line 7337 of /var/www/clients/client3/web8/web/sites/all/libraries/tcpdf/tcpdf.php).