upgrade vcenter 5.1 to 6.0 while error ：
[SSL: SSL_NEGATIVE_LENGTH] dh key too small (_ssl.c:661)' while validating ssl certificate
瑙ｅ喅鏂规: Make sure that the sso service is started before continuing"
can you check the keylength of the vcenter/rsa (ss0) certificates?
If they are 1024, they will have issues as similar to this below kb
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