Learn English – “It were students …’ or ‘It was students…’

At first glance, I would have said that the reason it is was is because there is always singular.

However, after looking at the linked question, I have to say that it agrees with the first item.

• There was an apple and an orange on the table. correct
• There were an apple and an orange on the table. questionable, non-standard?
• There was some apples and some oranges on the table. questionable, non-standard?
• There were some apples and some oranges on the table. correct
• There was an apple and some oranges on the table. correct
• There were an apple and some oranges on the table. questionable, non-standard?
• There was some apples and an orange on the table. incorrect, maybe non-standard?
• There were some apples and an orange on the table. correct

The reason that those cases of "there was" seems questionable and not entirely ungrammatical is because there is singular.

Most of their conversation was complaints about the company.
most of their conversation consisted of complaints about the company

is understandable with the meaning of multiple complaints within a single conversation, which is your desired intent.

Most of their conversation was complaint about the company.

follows the guideline

when the complement is shared by all constituents of the plural subject, it remains singular

You could also reword your sentence to be less ambiguous

Most of their conversation was complaining about the company.
Their conversation was mostly complaints about the company.

both imply many complaints in one conversation.