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In the paper, we investigate the Hyers–Ulam stability theorem of an Euler–Lagrange additive functional inequality
\[
\left\| \sum_{j=1}^{n} \left( \sum_{k=j}^{n} \lambda_k \right) f(x_j – x_{j-1}) \right\| \leq \left\| f\left( \sum_{j=1}^{n} \lambda_j x_j \right) \right\| + \varphi(x_1, \ldots, x_n),
\]
where \( x_0 = 0 \), \( n \geq 3 \), subject to control function \( \varphi \) in (non-Archimedean) Banach spaces.
