Unable to switch template for a single store in a multistore setup

[lmeyer]

I have a clean installation of 1.7.6.7. I added the classic-rocket theme under Appearance - Theme - Add a theme. I have multistore enabled, and just two basic stores. 

When I go into single store context, and then switch the theme only for this store, I get a Error 500, and afterwards, the FO of the two shops doesn't display correctly due to wrong/mixed CSS getting loaded.

The details of the error are here:

[Thu Aug 13 14:14:39.423113 2020] [***] AH01071: Got error '
PHP message: PHP Fatal error:  Uncaught Error: Class 'Tools' not found in /home/www-data/ps1767.sbgmb.net/classes/db/Db.php:794
Stack trace:
#0 /home/www-data/ps1767.sbgmb.net/config/alias.php(47): DbCore->escape('configuration', false)
#1 /home/www-data/ps1767.sbgmb.net/config/alias.php(52): pSQL('configuration')
#2 /home/www-data/ps1767.sbgmb.net/classes/Configuration.php(155): bqSQL('configuration')
#3 /home/www-data/ps1767.sbgmb.net/classes/Configuration.php(209): ConfigurationCore::loadConfiguration()
#4 /home/www-data/ps1767.sbgmb.net/classes/PrestaShopAutoload.php(171): ConfigurationCore::get('PS_DISABLE_OVER...')
#5 /home/www-data/ps1767.sbgmb.net/classes/PrestaShopAutoload.php(130): PrestaShopAutoload->generateIndex()
#6 [internal function]: PrestaShopAutoload->load('Tools')
#7 /home/www-data/ps1767.sbgmb.net/config/config.inc.php(85): spl_autoload_call('Tools')
#8 /home/www-data/ps1767.sbgmb.net/admin491uyu33e/index.php(40): require('/home/www-data/...')
#9 {main}
  thrown in /home/www-d...', referer: https://ps1767.sbgmb.net/admin491uyu33e/index.php/improve/design/themes/?_token=***

This seems like mysterious: how can the class `Tools` be missing ? The file is there ins `classes\Tools.php`. I also deleted `var\cache\prod\class_index.php`, but this does not help either.

Edited August 13, 2020 by lmeyer (see edit history)