The last election result from May was annulled after Austria's highest court upheld claims of procedural irregularities made by the narrowly-defeated far-right.
This necessitated fresh elections but this time there appear to be problems with glue on postal votes not sticking, making them invalid.
On Saturday independent ecologist Alexander Van der Bellen joined his rival in the vote, Norbert Hofer from the far-right, in saying he now expects a postponement.
"I don't believe that October 2 is possible any more," Van der Bellen, 72, told a news conference. "I hope that (the new election) can still take place this year."
Interior Minister Wolfgang Sobotka meanwhile made his clearest indication yet of a postponement, saying that "it does not look like" the problems can be resolved in time.
He is due to make an announcement on Monday.
Die Presse daily cited unnamed sources as saying that the government was looking at several possible dates in November, but that mid-December or even January were being considered.
Pushing the election back poses legal problems, however, and the government is considering drawing up special legislation allowing it to happen.
"This is an unbelievable disgrace for... the whole country," Die Presse said in an editorial entitled "Banana Republic".
Hofer, 45, officially launched his election campaign in Wels in northern Austria despite the likely postponement, hitting out at the "stupidity" of allowing mass immigration by "economic migrants".
The May 22 vote, a run-off after a first round in April, saw Van der Bellen narrowly beat Hofer by just 31,000 votes.
The FPOe has stoked concerns about recent record immigration, and should Hofer eventually win it would make Austria the first country in Europe since 1945 to elect a far-right president.
The role of the Austrian president is largely -- but not entirely -- ceremonial, and a victory by Hofer would be a major boost to Europe's other surging populist movements.