Platvoet said the by-election -- for 10 parliamentary seats -- was a "step forward" toward meeting international norms.
But he cautioned that his team was unable to get a full overview of the situation.
"Our committee was not of sufficient size to make a comprehensive assessment of the vote count and tabulating procedures," he said. "It expresses hope that the irregularities with the tabulation and protocols observed in the November election will not take place this time".
The election was in districts where results from the November 6 elections were annulled because of voting irregularities.
International observers said last year's election, in which the party of President Ilham Aliyev won the majority of seats, was not free or fair.
Arif Hajili, a deputy leader of the opposition Musavat party, said on May 13 that that the latest vote was still marred by violations.