Hi Jasmina
From what you say, your next door neighbour should be glad to have you there, as you are most considerate. It would seem that the strata manager is taking instructions from your neighbour, whereas he/she should be acting on behalf of the Executive Committee for the Strata Plan. The letter you received basically has no standing, whereas if you received a formal notice to comply with a bylaw, provided this has been properly drawn up, this may be enforced. Aggressive letters are not, in my opinion at least, a good way for a strata manager to make initial contact, as they do not encourage co-operation and are unenforceable. An initial contact in a friendly tone is a better way to go, before proceeding further. Hope things settle down for you.
DaveB