Critics have long argued that it is only a copy of a Rubens original that is known to have been painted between 1608 and 1609 for his Antwerp sponsor Nicolaas Rockox which then vanished after his death in 1640. Then it was not until in 1929 when it appeared again.

参考译文:批评家们长期认为，这幅画仅是鲁本斯原作的复制品。已知原作创作于1608至1609年之间，是由其安特卫普赞助人尼古拉斯·洛克索斯（Nicolaas Rockox）所创作，但洛克索斯在1640年去世后原作便突然消失，直到192年才再次出现。

备注(1) 第1个that 引导宾语从句(that引导名词性陈述分句)。

主句结构:Critics have long argued+that宾语从句

备注(2) 第2个that 引导定语从句，修饰前面名词original,关系代词that指代original并在定语从句作主语。

00001. 定语从句结构：original that is known to have been painted

‌特别备注：不定式完成体被动结构(to have been painted)表示不定式动作在谓语动词之前发生且完成，主语为不定式动作承受者，所以用不定式被动结构。

备注(3) 关系代词which 引导定语从句which then vanished after his death in 1640. 修饰前面名词original,关系代词which指代original并在定语从句作主语。

It was not until in 1929 when it appeared again. 

首先网友提供英语句子介词in应该去掉，变为下面句子

It was not until 1929 when it appeared again.

备注(1) 强调内容为“作时间状语的介词短语until 1929”。

备注(2) when用于非正式语体，正式语体用that替换后为下面句子。

It was not until 1929 that it appeared again.直到1929年它才再次出现。

备注(3) 把以上句子还原成非强调句型结构为

It didn’t appear again until 1929.它直到1929年才再次出现。

备注(4) 在非强调句中，not和until是分开的，在强调句not和until是靠在一起的。

类似以上句子例句如下供网友参考

【1】His father didn't come back until midnight 【非强调句】

        他的爸爸直到午夜才回来。

【2】It was not until midnight that his father came back【强调句】

       直到午夜他的爸爸才回来。

备注:以上是强调“时间状语(until midnight)”的强调句

【1】The first of the vitamins was not identified until 1911【非强调句】

第一种维生素直到1911年才被发现。

【2】It was not until 1911 that the first of the vitamins was identified.  【强调句】

直到1911年才发现了第一种维生素。

备注:以上是强调“时间状语(until 1911)”的强调句

it是形式主语，真主语是when it appeared again这句话