If one is expected to have unshakable faith in the Buddha, the Dhamma, and the Sangha and there is concern that an Ajahn's teachings may be inaccurate, is it still possible to take refuge?
Sangha here means the monks who preach inline with the teachings of the Buddha. So if a certain monk teaches something that goes against the teachings of the Buddha, it doesn't prevent you from taking refuge in the Triple Gem. You just have to find an authentic member of the Sangha to teach you. Even if you can't find one, you still can take refuge in the enlightened monks in the past.
The Sangha refers to the Noble Sangha. The Noble Sangha teach Noble Dhamma. The distinction between Noble & non-noble Dhamma is explained in MN 117:
And what is right view? Right view, I tell you, is of two sorts: There is right view with effluents, siding with merit, resulting in acquisitions [of becoming]; there is right view that is noble, without effluents, transcendent, a factor of the path.
And what is the right view with effluents, siding with merit, resulting in acquisitions? 'There is what is given, what is offered, what is sacrificed. There are fruits & results of good & bad actions. There is this world & the next world. There is mother & father. There are spontaneously reborn beings; there are contemplatives & brahmans who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.' This is the right view with effluents, siding with merit, resulting in acquisitions.
And what is the right view that is noble, without effluents, transcendent, a factor of the path? The discernment, the faculty of discernment, the strength of discernment, analysis of qualities as a factor for awakening, the path factor of right view in one developing the noble path whose mind is noble, whose mind is without effluents, who is fully possessed of the noble path. This is the right view that is noble, without effluents, transcendent, a factor of the path.
Please see below. The taking of refuge is in the "ideal" Sangha when considering the accuracy of the teachings, but in the "conventional" Sangha, when considering a debt of gratitude to the "conventional" sangha.
This means that you don't need to believe that a certain Ajahn or Mahathera is infallible in his accuracy of teaching, as the Ajahn or Mahathera can be considered to fall into the conventional definition of the word Sangha, since we cannot be absolutely certain of his spiritual attainment and virtues.
When unsure, use the Buddha's Words in the suttas as your teacher, as it is said in the Mahaparinibbana Sutta: "Now, if it occurs to any of you — 'The teaching has lost its authority; we are without a Teacher' — do not view it in that way. Whatever Dhamma & Vinaya I have pointed out & formulated for you, that will be your Teacher when I am gone."
From Thanissaro Bhikkhu's essay on Refuge:
The word Sangha, on the external level, has two senses: conventional and ideal. In its ideal sense, the Sangha consists of all people, lay or ordained, who have practiced the Dhamma to the point of gaining at least a glimpse of the Deathless. In a conventional sense, Sangha denotes the communities of ordained monks and nuns. The two meanings overlap but are not necessarily identical. Some members of the ideal Sangha are not ordained; some monks and nuns have yet to touch the Deathless. All those who take refuge in the Buddha, Dhamma, and Sangha become members of the Buddha's four-fold assembly (parisa) of followers: monks, nuns, male lay devotees, and female lay devotees. Although there's a widespread belief that all Buddhist followers are members of the Sangha, this is not the case. Only those who are ordained are members of the conventional Sangha; only those who have glimpsed the Deathless are members of the ideal Sangha. Nevertheless, any followers who don't belong to the Sangha in either sense of the word still count as genuine Buddhists in that they are members of the Buddha's parisa.
When taking refuge in the external Sangha, one takes refuge in both senses of the Sangha, but the two senses provide different levels of refuge. The conventional Sangha has helped keep the teaching alive for more than 2,500 years. Without them, we would never have learned what the Buddha taught. However, not all members of the conventional Sangha are reliable models of behavior. So when looking for guidance in the conduct of our lives, we must look to the living and recorded examples provided by the ideal Sangha. Without their example, we would not know (1) that Awakening is available to all, and not just to the Buddha; and (2) how Awakening expresses itself in real life.
On the internal level, the Buddha, Dhamma, and Sangha are the skillful qualities we develop in our own minds in imitation of our external models. For instance, the Buddha was a person of wisdom, purity, and compassion. When we develop wisdom, purity, and compassion in our own minds, they form our refuge on an internal level. The Buddha tasted Awakening by developing conviction, persistence, mindfulness, concentration, and discernment. When we develop these same qualities to the point of attaining Awakening too, that Awakening is our ultimate refuge. This is the point where the three aspects of the Triple Gem become one: beyond the reach of greed, anger, and delusion, and thus totally secure.
Also from the ATI Sangha page:
In the suttas the word sangha (lit. "group, assembly") is usually used in one of two ways: it refers either to the community of ordained monks and nuns (bhikkhu-sangha and bhikkhuni-sangha) or to the community of "noble ones" (ariya-sangha) — persons who have attained at least stream-entry, the first stage of Awakening. The definition (ariya-sangha)
"The Sangha of the Blessed One's disciples who have practiced well... who have practiced straight-forwardly... who have practiced methodically... who have practiced masterfully — in other words, the four types [of noble disciples] when taken as pairs, the eight when taken as individual types — they are the Sangha of the Blessed One's disciples: worthy of gifts, worthy of hospitality, worthy of offerings, worthy of respect, the incomparable field of merit for the world."
— AN 11.12