Are you confident that she's telling you the truth, with this story?
She had been previously given me the number of her Ex in case she had some sort of emergency, and I kept his number, until my phone crashed and deleted many things.
He knew of my existence previously, and she also recurred to him emotionally almost two months ago when we had that episode of break up. Ok, then this week she told him about the plans we have together, and it seems he has not stopped messaging her and acting in a jealous way. For example, today we were doing a video call and this guy kept sending her messages all the time, like "you fall for strangers that are abroad and are only interested in a EU passport" something I don't even care about.
I ask because she's demonstrated some triangulating already, and it wouldn't be out of character for her to lie to you about her ex's behavior.
To complete with, since she is an introvert and stayed so long with him, the apartment she is currently renting and paying through her own job, also appears on the rent contract under his name and he also had additional keys.
It's possible that she is still quite entangled with this ex...and it's possible that she is using you in some attempt to make him jealous. I'm not saying that's the case? But certainly, she's not demonstrating any skills in stable, adult partnership.
It is not like my feelings for her have changed. Today for example, she started having some sort of episode of thinking I was going to leave her, and we quickly clarified that I'm not. But I did mention that this bothers me. I even said, wouldn't it be better if I stay at a hotel? wouldn't it be better if she goes to me in Prague (as I'm planning on visiting my old study city) to which she declined saying that since she has to work yes or yes, if I don't stay with her we would have much less opportunity to be together.
Bottom line: she's not concerned about your feelings of unsafely, and she's not willing to consider accommodating you on this trip - which would be the most reasonable and realistic way to avoid tension with this ex.
Even that she gave you the number of her ex in case of an emergency - while you are still only an online contact, and you're in different countries (if I'm remembering correctly)...I'm very sorry to say it, but I think it's very possible that she's simply using you, either for her own dysfunctional needs for attention, or to serve her dysfunctional enmeshment with her ex.
It's possible that I'm completely wrong. But at a minimum, she's consistently demonstrating emotional manipulation, and that's a relationship habit that takes a lot of work to change. She doesn't seem fully aware of her own issues.
Edit to add:
I just would like to know, how to handle the ex husband aspect
She has not gone out with any other guy, and it is just this situation about her ex husband.
If nothing else, I can tell you this: Her situation with her ex-husband is HER situation, not yours. And I think the fact that she tells you he's angry and jealous, but also tells you to still come to the city where she is, and that his name is still on her lease? This means, she's not actually free and clear of that marriage, and it would be better for her to sort herself out before you go further with her, romantically.
Ex-husband situations are MUCH much more complex than any "other guy" situation can be. There's no 'just' about it, in my experience and opinion. It's OK that you don't know that - but I hope you can take it into consideration. I do hear you, saying how much you enjoy her company and how special your connection is. That can be true, and ALSO she can still be wrapped up in feelings for her ex. Better for the both of you if she gets some clarity for herself and more disconnection from the ex.
