definitely check the bolt length. If it is even (apparently) flush, that is too long. Grind a bit off the end of the bolt? Or, if there is a machined depression for the bolt head or nut to fit into, you could slip the fing around so the overall thickness of the spider arm/ring was a tad more.
Properly tightened, spacers should not be an issue. However, they do add some leverage to the mix where you don't want it, so they could be a contributing factor.
Riding fixed just adds a lot of stresses to the ring/crank interface. Crank those bolts down tight. Maybe put some locktite on them. Or, alternatively, add some grease to the threads, counterintuitively it lets you get them a bit tighter.
jim
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