I just found this thread because I was having the same issues with the trace tool. I believe I've found at least one of the causes of this problem. I had placed an arc segment to use for cutting a polygon. Immediately, I was unable to use the trace tool. If I panned my view until the arc segment was out of sight, I could use the trace tool again. I deleted the arc segment and again, was able to trace. Because I had to use that arc as my cutting edge, I resorted to generalizing the arc segment with a very small maximum allowable offset value (to retain the general shape of the arc). Trace became functional again. Sounds like arc (curve) segments may be a key to this issue... - Keith
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