Markus states he has had B12 injections which means he will already have had those tests including Serum Folate level. In fact he appears to say his Folate level is within normal range?
Threads like this are dangerous.
They are only dangerous if the armchair experts

, start dishing out advice and quoting shite from the web.
The Op post informs about something, I did not know about and it seems no one else did, and says get your GP to get you tested if you have undiagnosed symptoms that could be caused by it

. Which I had.
It was hijacked by those who think health is funny, cannot resist giving their view on every subject, usually negative, then start dishing out advice from the web, that is dangerous.
Only @Markus will know if he has been tested, I have had B12 tests several times over the last 3 years, but never Folate until last week

, it was a face to face and detailed description of recurring symptoms that lead to the full range of tests, but GP,s will never do that

, according to some who know every GP personally

.
The probiotics, which is the only thing I recommended to @Markus, along with being honest with his GP, and perhaps asking for tests, can do no harm to anyone, unless your are very unlucky and allergic to the contents.