Приложение долго не хотело загружаться, и я увидела предупреждение об ошибке.

Because приложение is a neuter noun in Russian. In the past tense, Russian verbs agree with the subject in gender and number if the subject is singular.

So:

• он хотел = he wanted
• она хотела = she wanted
• оно хотело = it wanted

Since приложение means application / app and is grammatically neuter, the sentence uses хотело.

Literally, yes. But this is a very natural way in Russian to describe something that was being difficult or refusing to work.

So Приложение долго не хотело загружаться means something like:

• The app wouldn’t load for a long time
• The app just didn’t want to load

This is a kind of mild personification, and it sounds very normal in everyday Russian. English does something similar with wouldn’t:

• My computer wouldn’t start
• The door won’t open

Russian often uses не хотеть + infinitive in that same spirit.

Here -ся shows that the app is undergoing the action itself: to load in the sense of to be loading / to load up.

Compare:

• загружать что-то = to load something
  
  • Я загружаю файл = I am uploading/loading a file
• загружаться = to load, to be loading
  
  • Приложение загружается = The app is loading

So the reflexive form is used because the app is not loading something else; the app itself is loading.

Because загружаться is the imperfective infinitive, and it fits the idea of an ongoing or repeated process.

In this sentence, the meaning is that the app was taking a long time and the loading process was not happening properly. That is naturally expressed with the imperfective:

• не хотело загружаться = wouldn’t load / wasn’t willing to load

If you used загрузиться instead, it would sound more focused on reaching the completed result to finish loading. That can work in some contexts, but here the ongoing process is more important.

Yes, absolutely. Приложение долго не загружалось is very natural and probably a bit more neutral.

The difference is mainly tone:

• Приложение долго не загружалось = The app wasn’t loading for a long time
  
  • straightforward, neutral description
• Приложение долго не хотело загружаться = The app wouldn’t load for a long time
  
  • more expressive, slightly conversational, with a sense of annoyance

So the original sentence sounds a little more human and emotional.

Because the speaker is female.

In the past tense, Russian verbs show gender when the subject is singular:

• я увидел = I saw (male speaker)
• я увидела = I saw (female speaker)

So увидела tells you that the person speaking is a woman.

Because увидеть / увидела is perfective, and it expresses a single completed event: the speaker noticed or saw the warning.

• увидела = saw, noticed
• видела = was seeing / saw in a more ongoing, descriptive, or habitual sense

In this sentence, the warning appears as one specific result or event, so увидела is the natural choice.

Because предупреждение об ошибке means a warning about an error.

The structure is:

• предупреждение = warning
• об ошибке = about an error

So the whole phrase means a warning message that concerns an error.

If you only said ошибка, that would mean an error itself, not a warning about an error.

Also, with предупреждение, Russian often uses a prepositional phrase with о / об when the meaning is warning about X.

Both о and об mean about, but об is used before certain vowel sounds because it is easier to pronounce.

So:

• о книге = about a book
• об ошибке = about an error

Since ошибке begins with a vowel sound, Russian uses об here.

Because after о / об, Russian normally uses the prepositional case.

The base form is:

• ошибка = error

After об, it becomes:

• об ошибке = about an error

So ошибке is the prepositional singular form of ошибка.

Because this sentence has two separate clauses, each with its own subject and verb:

1. Приложение долго не хотело загружаться
2. я увидела предупреждение об ошибке

Russian normally puts a comma before и when it joins two independent clauses like that.

So the comma is required here.

Долго means for a long time or a long time.

In this sentence it tells you that the app was refusing to load not just once or briefly, but over a noticeable period:

• Приложение долго не хотело загружаться = The app wouldn’t load for a long time

It modifies the whole loading situation.

Yes, in this context it clearly means app / application.

But приложение can also mean other things depending on context, for example:

• an application / app
• an appendix in a book or document
• an attachment in some contexts

Because the sentence talks about loading and an error warning, the software meaning is the obvious one here.