after many hours of trying to fix this mysterious issue I decided to reach out for help. Thanks to everyone willing to aid me.
When I send HTML emails from Woocommerce, those emails are received broken. It seems like Woocommerce adds many = characters spread randomly inside the code and text.
I did managed to locate something, that is probably causing the problem. When I go to email template (lets say customer-on-hold-order.php) and delete line with action woocommerce_email_order_details; It suddenly start working again. The received email is OK.
But I need to send the order details, deleting it permanently is a no go.
Also, the issue is with all woocommerce email templates, therefore I assume, that there must be some problem with email composition at the point where it adds the order details to the email.
And when I check the email preview within wordpress, its OK. And it breaks only after sending.
Things I tried thats worth mentioning (nothing helped):
1. disable all plugins and switch to basic theme
2. update woocommerce to most recent version
4. checked all files related to emails in woocommerce for any obvious issues
5. checked if WP emails has the same issues -> it doesnt; only woocommerce
6. checked if plain text emails are ok -> they are
7. tried multiple mail clients -> its broken everywhere, gmail handles it a bit better tho
8. tried different HTML templates via plugins -> still broken
9. I changed encoding just to be sure -> its not causing the issue