Steps to reproduce the issue

Installed an out-of-the-box Joomla 3.8.8 on localhost
Logged in to /administrator
Ok to update to 3.8.9

Expected result

Update succeeds

Actual result

Update failed at
http://justatest/administrator/index.php?option=com_joomlaupdate&task=update.finalise&e63f3ad56a496e6fa0a4db3b96627288=1

Fatal error: Uncaught Error: Class 'JClassLoader' not found in C:\www\justatest\libraries\cms.php on line 40
Error: Class 'JClassLoader' not found in C:\www\justatest\libraries\cms.php on line 40

After that both the front end and the back end fail with the same error.

System information (as much as possible)

Wampserver 3.1.3 on Windows 10
PHP 7.1.14
Apache 2.4.23
MySQL 5.7.14

There are several 3.8.8 sites on the same localhost, installed as Virtual Hosts, and they have worked without any issues.

Additional comments