Steps to reproduce the issue

• Install Joomla_4.0.0-alpha5-dev-Development-Full_Package.zip, 11 September 2018 02:00:22 UTC
• Installation hangs after entering credentials for database and starting installation afterwards.

Expected result

• Successful installation

Actual result

• Endless running spinner and errors in console:

Firefox 62.0 (64-Bit)/Win 10

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data: template.js:189:16
onSuccess
/installation/template/js/template.js:189:16
e.request/o.onreadystatechange
/media/system/js/core.min.js:1:8183

Vivaldi 1.15.1147.64 (Stable channel) (32-Bit)//Win10

index.php:1 Uncaught SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at onSuccess (template.js?1f470a31858053af25563fb8b1b5b13e:189)
    at XMLHttpRequest.o.onreadystatechange (core.min.js?1f470a31858053af25563fb8b1b5b13e:1)

System information (as much as possible)

See "System information" at #22121 (comment) (System information)