Hi again @Micha1972,
This may be helpful.
Magnesium glycinate is two molecules of the amino acid glycine attached to one atom of magnesium. Molecular weight 172.4. As such , it is 14.1 % elemental magnesium. SO ... your 600 mg of magnesium glycinate contains 84.6 mg of magnesium. That then is your daily supplement of elemental magnesium. Roughly one quarter of the total magnesium recommended daily intake, depending upon your adviser. Some recommend more.
Your blood serum measurement of 0 .8 mmol/L of magnesium is at the very bottom, or even below, the normal value. Magnesium is similar to calcium, in that your body adjusts its blood serum level, by donating from your cellular store of magnesium to the blood serum. i.e. you deplete your store to maintain the critical blood level. similarly with calcium, your store i.e. your bones, donates to the blood, under the influence of your endocrine system, If your store depletes, you may have consequences. The actual test to establish whether your body, including its store, has enough magnesium , ( i.e. not just the blood serum level ) is a Red Blood Cell magnesium test, which establishes whether the cell content is enough.
You might consider increasing your total elemental magnesium supplementation. You do seem to be low. Bear in mind my comment above, if you develop a loose bowel, split the dose, and /or change the type of supplement. But avoid Mag Oxide.
Perhaps you would be best asking your doc for an RBC test ? ... IF you do increase your magnesium, you should let him know, just in case you have any other interacting conditions. IF your symptoms do not decrease with increased magnesium, considering your now excellent level of vitamin D, he should be doing further investigations. Kidney function, PTH, dexa scan, etc etc.
Best of luck .
BTBH