Title says all. After filling in email and password, clicking the save to config field and logging in afterwards, the passwords wont get saved to config.

Would you be able to provide your log file?

Sure thing, sorry I forgot it earlier. https://pastebin.com/8vLJ9dGb
I think/hope thats all the important stuff from the log, its way too big to paste it all since I have MC running like 24/7

Please update your java 8 installation - Java was enforcing limitations on the allowed strength of cryptographic functions by default (for legal reasons) until Janauary 2018 (Java 8 Update 161). Your issue seems to be caused by this restriction.
If updating java is not an option for you (it shouldnt be) you can alternatively install the "Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files"

I have just now been informed that you have installed the java version mojang uses by default - i will therefore support that version an will write a fix shortly

Please test again with version 3.8.1