What the clause says and what it covers
The key clause appears on the paytable or in the game's own rules: "Malfunction voids all pays and plays". The regulator reviews it, the operator must show it, and it is policed for compliance. A fair game has safeguards to prevent malfunctions and provisions for recovering from them.
Under this rule, a fault in a round cancels the credits in that round--both the wagers and the wins. The affected stake is instead returned, and the game resumes without voided credits on the balance. It is not a compensation for an outcome (or round) lost, it is a non-payment of credits that were mistakenly received. It is aimed at the cases where a display fault, such as showing a non-win as a win, has caused an unearned payout.
Of course, a regulated game also has a provision saying what happens when a player claims an outcome was mistaken. Not every malfunctions means a win is voided: it is the display fault, not the result of a round, that the malfunction-exempt clause covers.
The screen is not the record
An online casino game runs in a remotely-hosted server operations, not in the app or browser on the phone. The phone, whether it displays an Android app, an iOS app or a mobile site, displays an output from the operator's game servers. The regulator requires this high level of security: the player is end-user and viewer, not operator and buyer.
The data for the game is held on the server. The phone screen images it, but it does not move or store it. Each round is run independently of the handset, and the records of what the server played must match the records the display shows. Where they disagree, the regulator looks at the server record. This is to guard against display faults, as well as to verify each round against the operator's internal records.
Of course, the phone offers screenshots, but they are snapshots. They show what the phone has displayed at an instant, and nothing more. They do not record the server output. The screenshot is not the record of the result or the reason for a decision: it is the display at a point in time, not a log in more than one entry. A screenshot does not replace the server records.
Where a malfunction does exist, though, the screenshot is important. Because it records what was visible at the time, it is evidence of what happened on the screen. It cannot serve to overturn the server record where the two disagree, but is one part of the details that may be needed for claiming a stake or voiding a win. An operator assesses a malfunction claim with a fair amount of investigation into what was viewable on or in the game, and a screenshot is part of the incident's record.
Who you complain to, in order
The stakes in some casino games can be high. Because they are so high, malfunctions are discussed in the interactive game's rules. The operator expects to receive about it, and by law must process it.
If a malfunction meant that a faulty result was displayed, and the operator does not correct it before the player requests it, the player files a ticket with the customer support account. All that the operator needs to decide the case is given in the ticket: the game, the stakes, the round and any screenshot from the online game's display that records the result.
Where the customer service department disallows the request, or does not return to the player within a small timeframe, the state regulator or tribal gaming authority is the next step. The operator holds a state licence or tribal authorisation. The licensing body requires proper internal structures to handle faulty rounds.
What to write down while it is on screen
As above, the regulator requires a malfunction record, with a date-time stamp, between the screen and the server. A screenshot supports the ticket to provide this proof, and is evidence in an investigation of the error's nature. It is not evidence that the server did pay or award anything, but of the actions the operator took.
The screenshot should be taken when the display fault first occurs, and is not the only factor. A claim also needs the following, to detail what was affecting the player's balance:
1. the game in question 2. the affected stake 3. the round, with its starting time 4. the round's ticket number (where one was assigned) 5. the screenshot
This information points the customer service to a match from the server logs--which is itself the source of the truth of the game's rounds, and the result. The screenshot does not match the server output, but details a mistake on a round or rounds that need checking. Live support should ask for these details when offered, but the player should record them at the point of error, in case they need to be repeated later.
Finally, a disconnection bears explanation: in a smooth operation, a display fault and a disconnection are two different errors, ruled differently in the fair game's terms. When that is a fault, the round is treated differently to how a malfunctioned one is. The rules show what they are for. If a playback error has occurred, the final server outputs are returned, and the incident is withdrawn, with the player's status as before the round.