misunderstanding and complaints at work

Hi All.  I'm sure this happens a lot, so please do redirect me if there's a discussion already.

TLDR: clients make complaints about my written 'tone', even though I think I'm being nice.  Any advice for getting my well-meaning but untrained office to stop taking their side? 

I've worked in my office for nearly 10 years.  4 years ago I disclosed I'm autistic.  They have been very slow to make accommodations, though they say very nice things to my face. 

I'm a fact checker and a client who was unable/unwilling to provide their evidence has just made a complaint about my tone in an email.  This is maybe the third time this has happened, at least, though from different clients.  Usually people say that I'm too brusque, although technically still polite.  This time I tried really hard to sound casual and jokey when laying out the requirements needed for proof.  The client has complained that now I sound condescending and patronising.  I can't win! 

Whenever one of these complaints comes in about me, my first reaction is 'why do they assume the worst?'.  If my employers know I'm autistic, how can I get them to stop taking the client's side and give me the benefit of the doubt - that maybe the client was on the defensive and chose to read my comments in the rudest voice they could imagine?  Maybe I, as an openly autistic person, am not trying to be mean or rude all the time? 

I guess this is as much a rant as asking for advice.  I feel like my job's in constant jeopardy even though technically I do everything right.  It's exhausting and really knocks my confidence back.