Problem is in intetkjønn substantiv. Why not "storte problem"?
It's plural. It's an -e ending no matter what, for most adjectives.
Yes, that's right, but it doesn't accept store problem, either. It insists on store problemer. When do inntetkjønn substantiver take "er" plurals and when don't they?
Here's a rule of thumb:
If a neuter noun has multiple syllables, you are allowed to add an -er ending in the plural.
If a neuter noun has one syllable, you are not allowed to add an -er ending in the plural.