Job Change being on fiktionsbescheinigung

As far as I understand (but please confirm that with your local "Ausländerbehörde"!), a "Fiktionsbescheinigung" is a temporary extension of your previous visa - with unchanged rights and restrictions. Thus if your previous visa allows work only for one specific company, you cannot change employer. Try applying for a work permit for the new employer!

I would add to what Beppi wrote that there are specific conditions to get a blue card. Even if a new job is sanctioned, one will have to apply again for the blue card depending on the information of new employment. It could be for example that the initial job paid high enough salary but the new one doesn't. Or if one is changing the field then the new job has to be one that qualifies one to get the blue card. By changing jobs you will in effect be invalidating your present blue card application.

If one has applied for a blue card then they should know the rules. You must be in contact with the officials regarding your application, so you should contact them and clarify the situation. To change jobs and not inform them, especially if the new job is not deemed appropriate for a blue card. then it could be seen as fraudulent for not withdrawing the application containing vital information that was no longer valid. Not saying one can't theoretically change jobs but do it correctly.

Here is a link that provides the basic info about blue cards:

https://www.auswaertiges-amt.de/en/visa … ard/606754