I've looked deeper into this issue. I am pretty sure that there is/was a race condition in the code. The only occurrence of this type of problem is indeed the setting of "connsperthread", therefore leaving the default value for this parameter solves this issue.
The issue is already part of NaviServer 4.99.0 (released July 2005). There is now a fix for this in the tip version in bitbucket, which will be included in the next release of NaviServer.