Can't access Install Tool on C4.8

I have two Contao 4.8 live installations (4.8.2 and 4.8.4), both on all-inkl, so I'm not really sure, whether it is about the provider or Contao.
I suddenly can't access Contao install tool (/contao/install) on both these installations. I keep getting Error 500.

The log in both cases says similar things, something like this:

Code:

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PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 532480 bytes) in /www/htdocs/.../var/cache/prod/ContainerJxx28xd/appContao_ManagerBundle_HttpKernel_ContaoKernelProdContainer.php on line 430
PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 532480 bytes) in /www/htdocs/.../vendor/composer/ClassLoader.php on line 444

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Try to increase the PHP memory limit. Edit your web/.htaccess and add e.g.

Code:

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php_value memory_limit 512M

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yeah, but why suddenly this problem?

Also, doesn't it mean, every time someone access this website, the server will allocate much more memory than it is needed? The problem is only encountered when the Install Tool is accessed...