Just as the title says, I'm looking for the meaning of this construction, like used in the following sentence:
Thanks in advance for help.
言う means to say. 言える means can say. 言えよう is the volitional form of 言える.
Therefore 言えよう means something along the lines of "It can be said" or "It could probably be said" and expresses a little bit of uncertainty in the statement. It may also be that the speaker doesn't want to express absolute agreement with the statement by using 言える in this way.
In context, 問題があると言えよう means something like "It can be said that there is a problem." or "It could probably be said that a problem exists".
言えよう = 言えるだろう
The よう part is the same as the ろう part of だろう, both originated from む. In modern Japanese, you will use だろう for all verbs, nouns and adjectives for this meaning. But in classic Japanese, you use different forms for them. Although slightly archaic, they are still used in academic writings.
But volitional verbs can't be used in this way.