shopware/platform

(SW6.2)
JS-Plugins of my themes only work in hot-reload-mode. If I do storefront:dev (or :build), I get error messages.

Chrome:

GET http://DOMAIN.TLD/theme/xxx/css/~scss/variables net::ERR_ABORTED 404 (Not Found)
all.js:8 Uncaught SyntaxError: Cannot use import statement outside a module

Firefox lists too a 404 for /variables and tells me also:

SyntaxError: import declarations may only appear at top level of a module

In this case my JS scripts won't run.

If I turn on hot-reload-mode (isHMRMode) and do a storefront:hot everything runs fine (and without error messages).

I don't understand those error messages and why it works in hot-reload-mode - but only in this mode.

I'm pretty sure I followed the how-to Shopware posted here: https://docs.shopware.com/en/shopware-platform-dev-en/theme-guide/javascript

My structure:

My /custom/plugins/THEMENAME/src/Resources/app/storefront/src/main.js looks like this:

import MyThemeFunctions from './my-theme-functions/my-theme-functions.plugin';

const PluginManager = window.PluginManager;

PluginManager.register('MyThemeFunctions', MyThemeFunctions);

And the following is my (shortened) /custom/plugins/THEMENAME/src/Resources/app/storefront/src/my-theme-functions/my-theme-functions.plugin.js:

import Plugin from 'src/plugin-system/plugin.class';
export default class MyThemeFunctions extends Plugin {
init.... etc..pp

}