Two things...
Which theme is that? If it's not a core theme, change to Boost (you may need to add $CFG->theme='boost'; in config.php)
Enable Debugging (again, you may need to copy the lines from section 7 of config-dist.php into config.php)
Third thing (!) - you could try a command-line upgrade.
Which theme is that? If it's not a core theme, change to Boost (you may need to add $CFG->theme='boost'; in config.php)
Enable Debugging (again, you may need to copy the lines from section 7 of config-dist.php into config.php)
Third thing (!) - you could try a command-line upgrade.