The Clan weakness of the Assamites was, starting with the Convention of Thorns till the modern nights, the Tremere Curse:
Due to the Tremere blood-curse, should an Assamite consume the blood of another Kindred, she suffers one automatic level of unsoakable lethal damage per blood point imbibed. Diablerie attempts result in automatic aggravated damage, one health level per point of permanent Willpower the victim possesses; the would-be diablerist gains no benefits (including Generation reduction) if he survives the process. In addition, Assamites must tithe some of the profits from their contracts to their sires or superiors (generally around 10 percent of all such earnings). 1
This is written to quite obviously make it impossible to drink any Vitae, be they Assamite, Camarilla or not. However, the Convention of Thorns itself, and the Treaty of Tyre both have a notable exception:
From this night forward, the Assamites shall henceforth no longer commit diablerie on members of other clans. The Assamites must commit themselves to this acceptance by a mark of assurance placed on them in the form of a Thaumaturgical limitation. All member of the Assamites shall become unable to drink freely of the vitae of other Kindred from now unto forever. 2
In fact, the Convention text is quite specific, that Assamites may freely Diablerize their own and those clans not in the Camarilla - which are for example Baali, Nagaraja, and Salubrii, but also in modern night's the Sabbat:
Assamites are also granted, out of respect of their beliefs, the freedom to commit diablerie within their own clan without restraint and the right to commit diablerie upon all Kindred not recognized as holding membership within the Camarilla.3
Three years after the convention, the Treaty of Tyre was signed, which is only fragmentary available. The extracts known lack that passage.
The Clan Assamite shall refrain from taking the Blood of other Kindred, and as surety of this provision shall submit themselves to a ritual to be administered by the Elders of the Clan Tremere, whereby Kindred Blood shall be made poisonous to them 4
Both of these do say other clans, not Assamites drinking from Assamites. Indeed the Convention explicitly carves the exception that they should be able to feed on each other as well as those vampires not in the Camarilla.
As a result, we got a Crunch-Fluff conflict: The Fluff says, that the Tremere should have only barred drinking from non-Assamite vampires, that are in the camarilla. The Crunch says, drinking from any Vampire, including Assamites, is poisonous.
So... did Tremere botch their Blood-Curse, just forgot to have that exclusion as dictated by the Convention of Thorns, or was this violation of the treaty text a deliberate act?
1 - V20 Core, p.49.
2 - V20 Anarchs Unbound, p.133, Left column: Convention of Thorns.
3 - V20 Anarchs Unbound, p.133, Left column: Convention of Thorns.
4 - V20 Anarchs Unbound, p.133, Right column: Treaty of Tyre.